fix(downloads): swap st.download_button for an HTML <a download> helper

Reported symptom: only the FIRST download button in a multi-button
row pops the browser save dialog. The second and third do nothing on
click. Affects every tool page that exposes (cleaned + audit + config)
downloads.

Root cause is ``st.download_button`` itself — when several render in
the same script pass, the click-to-bytes wiring on the browser side
mis-routes and only one button's data is actually exposed. Explicit
``key`` arguments don't fix it; ``use_container_width=True`` doesn't
help either; we confirmed this in the Text Cleaner reverts.

Replace the widget with a real ``<a download="file" href="data:...">``
anchor rendered via ``st.markdown(..., unsafe_allow_html=True)``.
Bypasses Streamlit's widget machinery entirely; behaves identically to
a native browser download. Side benefit: clicking it does NOT trigger
a script rerun, so other in-flight UI state survives.

New helper ``html_download_button`` lives in
``src/gui/components/_legacy.py`` (exported from ``components``). API:

    html_download_button(
        label, data,
        *, file_name, mime="application/octet-stream",
        disabled=False, help=None, use_container_width=True,
    )

Translation pattern applied across every tool page (and shared
``results_summary`` / ``config_panel`` widgets in ``_legacy.py``):

- ``st.download_button(`` -> ``html_download_button(``
- ``data=foo_bytes`` kwarg -> positional second arg
- ``key="..."`` -> dropped (helper has no widget identity)
- ``use_container_width=True`` -> dropped (default)
- ``disabled=`` and ``help=`` pass through unchanged
- Pre-computed byte buffers kept where they were

Total: 17 sites replaced (3 in Text Cleaner, 3 in Format
Standardizer, 3 in Fix Missing Values, 3 in Map Columns, 3 in
Automated Workflows, 2 in Find Duplicates page + 4 in shared
_legacy.py widgets used by Find Duplicates).

Caveat: data: URLs balloon by 33% (base64). Fine for tool output
sizes we ship; if a future result topped a few hundred MB we'd want a
Blob-URL fallback.

The marketing demo at src/gui/app_demo.py keeps its single
st.download_button — single button, no collision, no need to switch.

2008 tests pass.

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 (
     back_to_home_link,
     config_panel,
     hide_streamlit_chrome,
+    html_download_button,
     match_group_card,
     pickup_or_upload,
     require_feature_or_render_upgrade,
@@ -364,20 +365,17 @@ if uploaded is not None:
                             else b""
                         )
 
-                        st.download_button(
+                        html_download_button(
                             "Download Reviewed & Deduplicated CSV",
-                            data=reviewed_bytes,
+                            reviewed_bytes,
                             file_name="deduplicated_reviewed.csv",
                             mime="text/csv",
-                            key="dedup_dl_reviewed",
-                            use_container_width=True,
                         )
-                        st.download_button(
+                        html_download_button(
                             "Download Reviewed Removed Rows",
-                            data=reviewed_removed_bytes,
+                            reviewed_removed_bytes,
                             file_name="removed_reviewed.csv",
                             mime="text/csv",
-                            key="dedup_dl_reviewed_removed",
                             disabled=reviewed_removed_empty,
                             help=(
                                 "No rows were removed under the current "
@@ -385,7 +383,6 @@ if uploaded is not None:
                                 if reviewed_removed_empty
                                 else None
                             ),
-                            use_container_width=True,
                         )
 
             # Log entries