test(gui): add Streamlit AppTest layer (139 tests)

Until now every test ran against core or the CLI; the Streamlit GUI
was verified by hand. This commit adds tests/gui/ — 139 AppTest-
driven tests behind a 'gui' marker so the quick loop
(``pytest -m 'not gui'``) stays at 1777 tests / ~10s while
``pytest`` runs everything (1916 / ~14s).

Coverage:
- test_smoke.py (59): every page renders in EN and ES, expected
  substring present, sidebar selector mounted.
- test_chrome.py (18): language selector flips session state and
  re-renders; quit button + farewell strings localize; tool-card
  names use the active language.
- test_gate.py (9): require_normalization_gate no-op / warning /
  short-circuit / hash-mismatch invariants; warning + button
  localized.
- test_workflows.py (14): happy path per Ready tool — stash
  upload, render, find primary action, verify result lands in
  session state.
- test_dedup_review.py (8): Accept All / Reject All / Clear
  Decisions wire through to review_decisions; apply_review_decisions
  semantics (keep-all, merge, column override).
- test_advanced_panels.py (15): config_panel widget defaults and
  options (algorithm, threshold, survivor rule, merge, multiselects,
  config save/load).
- test_errors.py (4): garbage / empty / single-column uploads don't
  crash; duplicate-target mapping raises InputValidationError.
- test_findings_panel.py (12): driven via a small standalone harness
  page so we test the component without faking a file_uploader. EN
  + ES strings, per-tool grouping, open-tool button label, untargeted
  expander, severity summary.

Shared infrastructure in tests/gui/conftest.py:
- ``stash_upload`` / ``stash_upload_without_gate`` — populate
  session_state to pre-pass or block the gate.
- ``with_language`` — set ``ui_lang`` before run().
- ``collected_text`` — flatten title/caption/markdown/etc. into
  one string for substring assertions.
- Auto-marking: every test in tests/gui/ gets ``@pytest.mark.gui``
  via ``pytest_collection_modifyitems``, so the marker isn't
  per-test boilerplate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/gui/test_advanced_panels.py

@@ -0,0 +1,194 @@
+"""Advanced-options panel tests.
+
+``config_panel`` (in ``src.gui.components``) is the dedup-page's
+expander that houses every per-column / per-strategy knob. It's the
+densest single widget surface in the GUI, so a session-state key drift
+in there cascades into every dedup session.
+
+We exercise it via the Deduplicator page (rendering ``config_panel``
+in isolation requires a fake Streamlit context). The page provides
+the surrounding state; we poke widgets and verify their effects.
+"""
+
+from __future__ import annotations
+
+import pandas as pd
+import pytest
+
+from .conftest import stash_upload
+
+
+GATED_PAGE = "1_Deduplicator"
+
+
+def _render_page(app_factory, small_csv_bytes):
+    app = app_factory(GATED_PAGE)
+    stash_upload(app, name="messy.csv", data=small_csv_bytes)
+    app.run()
+    return app
+
+
+# ---------------------------------------------------------------------------
+# Expander presence + collapsed state
+# ---------------------------------------------------------------------------
+
+class TestAdvancedExpander:
+    def test_advanced_options_expander_renders(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        labels = [e.label for e in app.expander]
+        assert any("Advanced Options" in lbl for lbl in labels), (
+            f"Advanced Options expander missing; expanders: {labels}"
+        )
+
+
+# ---------------------------------------------------------------------------
+# Algorithm selector
+# ---------------------------------------------------------------------------
+
+class TestAlgorithmSelector:
+    """The fuzzy-algorithm dropdown drives ``Algorithm.{LEVENSHTEIN,
+    JARO_WINKLER, TOKEN_SET_RATIO}`` on every column. Default value
+    must be jaro_winkler — the strong-key build_default_strategies
+    assumes it."""
+
+    def test_default_algorithm_is_jaro_winkler(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        # Find the selectbox by label.
+        algo_boxes = [
+            sb for sb in app.selectbox
+            if sb.label == "Fuzzy algorithm"
+        ]
+        assert len(algo_boxes) == 1
+        assert algo_boxes[0].value == "jaro_winkler"
+
+    def test_algorithm_options_complete(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        algo = next(sb for sb in app.selectbox if sb.label == "Fuzzy algorithm")
+        assert set(algo.options) == {
+            "jaro_winkler", "levenshtein", "token_set_ratio",
+        }
+
+
+# ---------------------------------------------------------------------------
+# Threshold slider
+# ---------------------------------------------------------------------------
+
+class TestThresholdSlider:
+    def test_default_threshold_is_85(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        thresholds = [s for s in app.slider if "Similarity" in (s.label or "")]
+        assert len(thresholds) == 1
+        assert thresholds[0].value == 85
+
+    def test_threshold_bounds(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        thr = next(s for s in app.slider if "Similarity" in (s.label or ""))
+        assert thr.min == 50
+        assert thr.max == 100
+
+
+# ---------------------------------------------------------------------------
+# Survivor rule selector
+# ---------------------------------------------------------------------------
+
+class TestSurvivorSelector:
+    def test_default_is_first(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        surv = next(
+            sb for sb in app.selectbox
+            if sb.label == "Survivor rule"
+        )
+        assert surv.value == "first"
+
+    def test_all_four_rules_offered(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        surv = next(sb for sb in app.selectbox if sb.label == "Survivor rule")
+        assert set(surv.options) == {
+            "first", "last", "most-complete", "most-recent",
+        }
+
+    def test_selecting_most_recent_does_not_crash(
+        self, app_factory, small_csv_bytes,
+    ):
+        """When ``most-recent`` is chosen the page should reveal a
+        Date column dropdown. Pin the no-crash invariant."""
+        app = _render_page(app_factory, small_csv_bytes)
+        surv = next(sb for sb in app.selectbox if sb.label == "Survivor rule")
+        surv.select("most-recent").run()
+        assert not app.exception
+
+
+# ---------------------------------------------------------------------------
+# Merge checkbox
+# ---------------------------------------------------------------------------
+
+class TestMergeCheckbox:
+    def test_merge_default_off(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        merge_boxes = [c for c in app.checkbox if c.label == "Merge mode"]
+        assert len(merge_boxes) == 1
+        assert merge_boxes[0].value is False
+
+    def test_toggling_merge_does_not_crash(
+        self, app_factory, small_csv_bytes,
+    ):
+        app = _render_page(app_factory, small_csv_bytes)
+        merge = next(c for c in app.checkbox if c.label == "Merge mode")
+        merge.check().run()
+        assert not app.exception
+        # After checking, the value persists in session_state via the
+        # widget's own key.
+
+
+# ---------------------------------------------------------------------------
+# Column multiselects
+# ---------------------------------------------------------------------------
+
+class TestColumnMultiselects:
+    """Match-on / Strong-keys / Fuzzy multiselects use ``st.multiselect``
+    on every column. Empty default = auto-detect."""
+
+    def test_three_multiselects_present(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        labels = {m.label for m in app.multiselect}
+        assert {"Match on columns", "Strong keys", "Fuzzy columns"} <= labels
+
+    def test_defaults_are_empty(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        for ms in app.multiselect:
+            if ms.label in {
+                "Match on columns", "Strong keys", "Fuzzy columns",
+            }:
+                assert ms.value == [], (
+                    f"{ms.label!r} default should be []; got {ms.value}"
+                )
+
+    def test_options_match_dataframe_columns(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        df_cols = list(app.session_state["df"].columns)
+        for ms in app.multiselect:
+            if ms.label in {
+                "Match on columns", "Strong keys", "Fuzzy columns",
+            }:
+                assert list(ms.options) == df_cols
+
+
+# ---------------------------------------------------------------------------
+# Save / Load config
+# ---------------------------------------------------------------------------
+
+class TestConfigSaveLoadButtons:
+    def test_save_settings_button_present(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        labels = [b.label for b in app.button]
+        assert any("Save current settings" in lbl for lbl in labels)
+
+    def test_config_file_uploader_present(self, app_factory, small_csv_bytes):
+        app = _render_page(app_factory, small_csv_bytes)
+        # AppTest exposes uploaders via ``app.file_uploader``. There
+        # are two: the main file (pickup-or-upload) and the config
+        # JSON. Make sure the config one is there.
+        labels = [u.label for u in app.file_uploader]
+        assert any("Load config profile" in lbl for lbl in labels), (
+            f"config uploader missing; uploaders: {labels}"
+        )