fix(home): widget's "✕" Remove now actually removes the file

Reported: on the Home page after uploading data files, the Remove
buttons "on the right side" did nothing — the file kept showing up in
the list. That was the file_uploader widget's BUILT-IN ✕ icons (the
ones inside the uploader's chrome, on the right of each file row),
not our custom "Remove" buttons further down — the custom ones have
worked correctly since 84e4665.

Cause: ``_home_page`` deliberately treated the widget as add-only and
never honored widget-side removals. The reasoning, per the prior
comment, was that navigation can remount the widget with value ``[]``
— a render-time sync would then wipe ``home_uploads``. Real, but the
side effect was that the widget's own ✕ appeared to do nothing: the
file vanished from the widget chrome, stayed in ``home_uploads``, and
re-rendered immediately in the custom list below.

Fix: hook the file_uploader's ``on_change`` callback to reconcile
``home_uploads`` against the widget's current value. Streamlit's
``on_change`` fires ONLY on user-initiated value changes; the
remount-induced ``[]`` reset doesn't trigger it, so the stash still
survives navigation. Removals from the callback also drop the file's
findings entry and clear the singular ``home_uploaded_*`` keys when
the active upload was removed — matching the custom-button path.

The custom "Remove" buttons further down keep working unchanged; the
existing AppTest path through ``_home_remove_<sha1>`` still removes
exactly the file clicked. 2220 tests pass.

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

src/gui/_home.py

@@ -36,6 +36,43 @@ class _StashedUpload:
         return self._data
 
 
+def _sync_uploader_to_home_uploads() -> None:
+    """``on_change`` callback for the home-page file_uploader.
+
+    Reconciles ``home_uploads`` (our persistent stash) with the widget's
+    current value: adds newly-uploaded files, and drops files the user
+    explicitly removed via the widget's built-in "✕" button. Per
+    Streamlit semantics ``on_change`` only runs for user-initiated
+    value changes, so the navigation-induced ``[]`` reset never reaches
+    here — the stash survives intact across page switches.
+    """
+    from src.audit import log_event
+
+    widget_files = st.session_state.get("home_upload") or []
+    home_uploads: dict = st.session_state.setdefault("home_uploads", {})
+    findings: dict = st.session_state.setdefault("home_findings_by_file", {})
+
+    widget_names = {f.name for f in widget_files}
+
+    for f in widget_files:
+        if f.name not in home_uploads:
+            home_uploads[f.name] = {"bytes": f.getvalue(), "size": f.size}
+            log_event("upload", f"Uploaded {f.name}", filename=f.name, bytes=f.size)
+
+    for name in list(home_uploads.keys()):
+        if name not in widget_names:
+            del home_uploads[name]
+            findings.pop(name, None)
+            log_event("upload", f"Removed {name}", filename=name)
+            if st.session_state.get("home_uploaded_name") == name:
+                st.session_state.pop("home_uploaded_name", None)
+                st.session_state.pop("home_uploaded_size", None)
+                st.session_state.pop("home_uploaded_bytes", None)
+
+    st.session_state["home_uploads"] = home_uploads
+    st.session_state["home_findings_by_file"] = findings
+
+
 def _home_page() -> None:
     """Render the home page — multi-file upload + per-file analysis.
 
@@ -79,35 +116,26 @@ def _home_page() -> None:
     # Source of truth for uploaded files. dict[name -> {"bytes", "size"}].
     home_uploads: dict = st.session_state.setdefault("home_uploads", {})
 
-    # File uploader — for ADDING new files only. On every render we
-    # merge widget-returned files INTO home_uploads but never remove
-    # via the widget. (Widget state can return ``[]`` after navigation,
-    # which we deliberately don't treat as "user cleared their files".)
-    new_files = st.file_uploader(
+    # File uploader — syncs into home_uploads via on_change. We deliberately
+    # do NOT merge widget state into home_uploads at render time: navigation
+    # can remount the widget with value ``[]``, and a render-time merge
+    # would mistakenly leave home_uploads untouched while the user thinks
+    # they're looking at empty state.
+    #
+    # ``on_change`` fires ONLY on user-initiated value changes (uploads
+    # and the widget's built-in "✕" remove). It does NOT fire on the
+    # remount-induced reset. That lets us treat the callback as ground
+    # truth for both adds AND removes — fixing the previous bug where
+    # the widget's "✕" appeared to do nothing because the file persisted
+    # in home_uploads and immediately re-rendered in the list below.
+    st.file_uploader(
         t("upload.uploader_label_multi"),
         type=["csv", "tsv", "xlsx", "xls"],
         accept_multiple_files=True,
         key="home_upload",
         help=t("upload.uploader_help"),
+        on_change=_sync_uploader_to_home_uploads,
     )
-    if new_files:
-        from src.audit import log_event
-        changed = False
-        for f in new_files:
-            if f.name not in home_uploads:
-                home_uploads[f.name] = {
-                    "bytes": f.getvalue(),
-                    "size": f.size,
-                }
-                changed = True
-                log_event(
-                    "upload",
-                    f"Uploaded {f.name}",
-                    filename=f.name,
-                    bytes=f.size,
-                )
-        if changed:
-            st.session_state["home_uploads"] = home_uploads
 
     # Persistent file list with per-file remove buttons. We render this
     # ourselves rather than trusting Streamlit's widget chrome because