feat(gui): tool pages pick up the home-page upload via session_state

Closes the last UX gap from the analyzer review: each tool page had its
own st.file_uploader, so users had to upload the same file twice (once
on the home page for analysis, once on each tool page).

components.pickup_or_upload(label, key, types) returns either:
  - a _StashedUpload shim wrapping the home-page bytes (when present and
    the user hasn't asked for a different file on this page), or
  - the standard st.file_uploader (when nothing is stashed or the user
    clicked "Use a different file").

_StashedUpload duck-types Streamlit's UploadedFile (.name, .size,
.getvalue(), .read()) so existing tool-page code consumes it without
changes. A "Use a different file" button per page sets a session-state
override flag; a "Switch back to upload-screen file" button clears it.

Wired into 2_Text_Cleaner.py and 1_Deduplicator.py — the two pages with
working uploaders today. The remaining stub pages adopt it when they're
implemented; the helper is the public surface they'll use.

Verified by smoke-launching streamlit headless and curling the home,
text-cleaner, and deduplicator routes — all return 200 with no errors
in the server log.

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

src/gui/pages/1_Deduplicator.py

@@ -21,6 +21,7 @@ from src.gui.components import (
     config_panel,
     hide_streamlit_chrome,
     match_group_card,
+    pickup_or_upload,
     results_summary,
 )
 
@@ -56,11 +57,11 @@ st.caption("Find and remove duplicate rows in CSV, delimited text, and Excel fil
 # File upload
 # ---------------------------------------------------------------------------
 
-uploaded = st.file_uploader(
-    "Upload CSV or Excel file",
-    type=["csv", "tsv", "xlsx", "xls"],
-    help="Supports CSV, TSV, and Excel files. Encoding and delimiters are auto-detected.",
+uploaded = pickup_or_upload(
+    label="Upload CSV or Excel file",
     key="dedup_file_upload",
+    types=["csv", "tsv", "xlsx", "xls"],
+    help="Supports CSV, TSV, and Excel files. Encoding and delimiters are auto-detected.",
 )
 
 if uploaded is not None: