fix(text-cleaner): make all three download buttons actually fire

Only "Download cleaned CSV" was working; "Download changes audit" and
"Download config JSON" did nothing on click.

The symptom is the classic Streamlit footgun for multiple
``st.download_button`` widgets in adjacent columns: without an explicit
``key`` argument the auto-derived widget IDs can collide, especially
when one button is conditionally rendered, and only the first button
in source order actually fires on click. Same goes for unstable
``data`` bytes recomputed inside the ``with col:`` block — the widget
identity can drift between renders.

Robustness pattern applied:
- Compute all three byte buffers up front, outside the columns, so the
  ``data`` parameter is the same object across reruns.
- Pass an explicit unique ``key`` ("textclean_dl_cleaned" /
  "textclean_dl_changes" / "textclean_dl_config") to each button.
- Render the changes button unconditionally with ``disabled=True`` and
  a help tooltip when ``result.changes.empty`` — instead of hiding it.
  Layout stays steady and the empty case is self-explanatory.
- ``use_container_width=True`` so the three buttons size identically
  inside their columns.

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

src/gui/pages/2_Text_Cleaner.py

@@ -328,35 +328,55 @@ else:
 # ---------------------------------------------------------------------------
 # Downloads
 # ---------------------------------------------------------------------------
+#
+# All three byte buffers are prepared up front (outside the columns) so
+# each ``st.download_button`` sees stable ``data`` across reruns and an
+# explicit ``key`` — without those, Streamlit auto-derived widget IDs
+# can collide for multiple download_buttons in adjacent columns and
+# only the first one actually fires on click. The empty-changes case
+# now renders a disabled button (rather than vanishing) so the layout
+# stays steady and the user understands why nothing's available.
 
 st.divider()
 stem = Path(st.session_state.get("textclean_input_name", "input")).stem
 
+cleaned_bytes = result.cleaned_df.to_csv(index=False).encode("utf-8-sig")
+changes_bytes = (
+    result.changes.to_csv(index=False).encode("utf-8-sig")
+    if not result.changes.empty
+    else b""
+)
+config_bytes = json.dumps(options.to_dict(), indent=2).encode("utf-8")
+
 dl_a, dl_b, dl_c = st.columns(3)
 with dl_a:
-    cleaned_bytes = result.cleaned_df.to_csv(index=False).encode("utf-8-sig")
     st.download_button(
         "Download cleaned CSV",
         data=cleaned_bytes,
         file_name=f"{stem}_cleaned.csv",
         mime="text/csv",
+        key="textclean_dl_cleaned",
+        use_container_width=True,
     )
 with dl_b:
-    if not result.changes.empty:
-        changes_bytes = result.changes.to_csv(index=False).encode("utf-8-sig")
     st.download_button(
         "Download changes audit",
         data=changes_bytes,
         file_name=f"{stem}_changes.csv",
         mime="text/csv",
+        key="textclean_dl_changes",
+        disabled=result.changes.empty,
+        help="No changes to audit." if result.changes.empty else None,
+        use_container_width=True,
     )
 with dl_c:
-    config_bytes = json.dumps(options.to_dict(), indent=2).encode("utf-8")
     st.download_button(
         "Download config JSON",
         data=config_bytes,
         file_name="text_clean_config.json",
         mime="application/json",
+        key="textclean_dl_config",
+        use_container_width=True,
     )
 
 st.divider()