fix(pdf): stamp scan timestamp once; restores Saved-to-path banner

After swapping to ``html_download_button`` the user noticed the
"✓ Saved to <path>" + 📂 Open Downloads folder pair never
appeared. The helper itself is fine — every other tool shows
those affordances correctly. Bug was specific to the PDF page.

The download button's file_name was being computed with a fresh
``datetime.now().strftime(...)`` on every render. The helper
builds its session-state keys from
``f"_dl_btn_{file_name}_{digest}"`` so the keys silently drift
every second. After the click and rerun, the helper looks up
the saved_key for the NEW file_name, finds nothing in
session_state (the click had written to the OLD key), and skips
the success banner.

Fix: stamp the timestamp once when scan completes, store it in
``K_TIMESTAMP``, and reuse it for the download filename. The
filename stays stable across reruns, so the helper's keys are
stable, so the saved-path banner renders correctly on the post-
click rerun.

Also clear ``K_TIMESTAMP`` on Clear-all-files so a new scan
gets a fresh stamp.

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

src/gui/pages/10_PDF_Extractor.py

@@ -65,6 +65,13 @@ render_sticky_footer()
 K_ROWS = "pdf_scan_rows"
 K_WARNINGS = "pdf_scan_warnings"
 K_SOURCE_COUNT = "pdf_scan_source_count"
+# Stamped once at scan time. The download button's file_name
+# embeds this so the user gets a unique-per-scan filename — but
+# crucially, the value is stable across reruns triggered by
+# unrelated widget interactions (otherwise the html_download_button
+# helper's session-state key drifts every second and the
+# "Saved to <path>" banner never gets to render).
+K_TIMESTAMP = "pdf_scan_timestamp"
 # ``pdf_uploads`` is the persistent stash of uploaded PDFs (dict
 # keyed by filename → {"bytes": ..., "size": ...}). It survives
 # Streamlit reruns and navigation away from the page. The
@@ -386,7 +393,7 @@ with c_clear:
     ):
         st.session_state[K_UPLOADS] = {}
         st.session_state[K_UPLOAD_COUNTER] = upload_counter + 1
-        for k in (K_ROWS, K_WARNINGS, K_SOURCE_COUNT):
+        for k in (K_ROWS, K_WARNINGS, K_SOURCE_COUNT, K_TIMESTAMP):
             st.session_state.pop(k, None)
         log_event(
             "upload",
@@ -441,6 +448,7 @@ if scan_clicked and pdf_uploads:
     st.session_state[K_ROWS] = all_rows
     st.session_state[K_WARNINGS] = all_warnings
     st.session_state[K_SOURCE_COUNT] = n_files
+    st.session_state[K_TIMESTAMP] = datetime.now().strftime("%Y%m%d-%H%M%S")
 
     log_event(
         "tool_run",
@@ -606,7 +614,10 @@ else:
         if selected.empty:
             st.button("Download CSV", disabled=True)
         else:
-            ts = datetime.now().strftime("%Y%m%d-%H%M%S")
+            # Reuse the timestamp stamped when this scan finished —
+            # stable across reruns so the download helper's button
+            # key doesn't drift every second.
+            ts = st.session_state.get(K_TIMESTAMP) or "results"
             # Default: drop the internal columns from the download.
             keep_default = [
                 c for c in selected.columns