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_chrome.py

@@ -0,0 +1,181 @@
+"""Chrome tests — language selector, hide_streamlit_chrome, quit flow.
+
+These verify the GUI plumbing that every page depends on. Failures here
+cascade into every other page, so they run cheap and run first
+(alphabetical name ordering after smoke).
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from .conftest import collected_text, with_language
+
+
+# ---------------------------------------------------------------------------
+# hide_streamlit_chrome mounts the selector
+# ---------------------------------------------------------------------------
+
+class TestHideChromeMountsSelector:
+    """``hide_streamlit_chrome()`` is the one place the language selector
+    is mounted. Every page that hides chrome (= every page) must get
+    exactly one sidebar selectbox with the i18n label."""
+
+    def test_home_has_one_sidebar_selectbox(self, home_app):
+        home_app.run()
+        # Only one selectbox in the sidebar today; if a page adds
+        # another, this becomes a weaker bound.
+        assert len(home_app.sidebar.selectbox) == 1, (
+            "expected exactly one sidebar selectbox (the language picker); "
+            f"got {len(home_app.sidebar.selectbox)}"
+        )
+
+    def test_selector_label_is_localized(self, home_app):
+        with_language(home_app, "es")
+        home_app.run()
+        labels = [sb.label for sb in home_app.sidebar.selectbox]
+        assert "Idioma" in labels, (
+            f"Spanish selector should be labelled 'Idioma'; got {labels}"
+        )
+
+    def test_selector_label_english_default(self, home_app):
+        home_app.run()  # no with_language → default = en
+        labels = [sb.label for sb in home_app.sidebar.selectbox]
+        assert "Language" in labels
+
+
+# ---------------------------------------------------------------------------
+# Language selector switches session state
+# ---------------------------------------------------------------------------
+
+class TestLanguageSwitch:
+    """Picking 'es' in the selector flips ``st.session_state['ui_lang']``
+    and re-renders the page with Spanish strings on the next run."""
+
+    def test_default_language_is_english(self, home_app):
+        home_app.run()
+        # AppTest's session_state proxy doesn't implement .get(); use
+        # membership check + attribute access. Absence == default ("en").
+        lang = home_app.session_state["ui_lang"] if "ui_lang" in home_app.session_state else "en"
+        assert lang == "en"
+        text = collected_text(home_app)
+        assert "Data Cleaning Mastery" in text
+
+    def test_selecting_spanish_persists_in_session(self, home_app):
+        home_app.run()
+        selector = home_app.sidebar.selectbox[0]
+        selector.select("es").run()
+        assert home_app.session_state["ui_lang"] == "es"
+
+    def test_selecting_spanish_re_renders_in_spanish(self, home_app):
+        home_app.run()
+        selector = home_app.sidebar.selectbox[0]
+        selector.select("es").run()
+        text = collected_text(home_app)
+        assert "Maestría" in text, (
+            "after selecting Spanish, the home title should switch to "
+            f"'🧹 DataTools — Maestría…'; got:\n{text[:300]}"
+        )
+
+    def test_selecting_back_to_english_reverts(self, home_app):
+        # Start in Spanish, then flip back.
+        with_language(home_app, "es")
+        home_app.run()
+        assert "Maestría" in collected_text(home_app)
+
+        selector = home_app.sidebar.selectbox[0]
+        selector.select("en").run()
+        text = collected_text(home_app)
+        assert "Data Cleaning Mastery" in text
+        assert "Maestría" not in text
+
+
+# ---------------------------------------------------------------------------
+# Footer + page_title localization
+# ---------------------------------------------------------------------------
+
+class TestLocalizedChrome:
+    """A spot-check on the parts of the chrome that aren't the selector:
+    the bottom footer caption and the home-page hero text. Other strings
+    are pinned indirectly by ``TestEveryPageRenders.test_expected_*``."""
+
+    def test_footer_english(self, home_app):
+        home_app.run()
+        text = collected_text(home_app)
+        assert "Your data never leaves" in text
+
+    def test_footer_spanish(self, home_app):
+        with_language(home_app, "es")
+        home_app.run()
+        text = collected_text(home_app)
+        assert "Tus datos nunca salen" in text
+
+    def test_upload_section_heading_localizes(self, home_app):
+        with_language(home_app, "es")
+        home_app.run()
+        text = collected_text(home_app)
+        # ``📤 Sube un archivo para empezar`` from the es pack.
+        assert "Sube un archivo" in text
+
+
+# ---------------------------------------------------------------------------
+# Quit / Close page
+# ---------------------------------------------------------------------------
+
+class TestQuitButtonRenders:
+    """The Close page must show the localized title, body, and the
+    Close-the-app button. We don't actually click the button — that
+    would call ``os._exit(0)`` and kill the test process. We only
+    assert the button is present and its label is localized."""
+
+    def test_close_page_english(self, app_factory):
+        app = app_factory("99_Close")
+        app.run()
+        text = collected_text(app)
+        assert "Close DataTools" in text
+        labels = [b.label for b in app.button]
+        assert any("Close the app" in lbl for lbl in labels), (
+            f"Close-the-app button missing; buttons: {labels}"
+        )
+
+    def test_close_page_spanish(self, app_factory):
+        app = app_factory("99_Close")
+        with_language(app, "es")
+        app.run()
+        text = collected_text(app)
+        assert "Cerrar DataTools" in text
+        labels = [b.label for b in app.button]
+        assert any("Cerrar la app" in lbl for lbl in labels), (
+            f"Spanish Close button missing; buttons: {labels}"
+        )
+
+    def test_close_body_describes_unsaved_work_warning_es(self, app_factory):
+        app = app_factory("99_Close")
+        with_language(app, "es")
+        app.run()
+        text = collected_text(app)
+        assert "trabajo sin guardar" in text
+
+
+# ---------------------------------------------------------------------------
+# Tool cards use localized names on the home grid
+# ---------------------------------------------------------------------------
+
+class TestHomeToolGridLocalization:
+    """The home grid pulls tool display names through ``tool_name()`` in
+    ``tools_registry``. The Spanish pack provides translations for every
+    tool id; a regression in that wiring would make Spanish users see
+    English names. Pin a few representative ones."""
+
+    @pytest.mark.parametrize("needle", [
+        "Eliminador de duplicados",
+        "Limpiador de texto",
+        "Estandarizador de formatos",
+        "Gestor de valores faltantes",
+        "Mapeador de columnas",
+    ])
+    def test_es_tool_name_on_home_grid(self, home_app, needle):
+        with_language(home_app, "es")
+        home_app.run()
+        text = collected_text(home_app)
+        assert needle in text, f"missing localized tool name {needle!r}"