chore: migrate use_container_width → width (Streamlit deprecation)

``use_container_width`` is being removed after 2025-12-31. Streamlit
log was flooding the terminal with the deprecation notice on every
rerun. Mechanical sweep:

  use_container_width=True   →  width="stretch"
  use_container_width=False  →  width="content"

51 call sites across 11 page files + ``app_demo.py``. Also renamed
the ``local_download_button`` helper's ``use_container_width`` kwarg
to ``width`` (default ``"stretch"``); it has no external callers
passing the old name, so this is a safe rename.

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

src/gui/components/_legacy.py

@@ -215,7 +215,7 @@ def _render_diagnostics_sidebar() -> None:
                 "📂  Open log folder",
                 key="_diag_open_logs",
                 type="secondary",
-                use_container_width=True,
+                width="stretch",
             ):
                 opened = _open_in_file_manager(audit_log_dir(), select=log_path)
                 if not opened:
@@ -448,7 +448,7 @@ def local_download_button(
     mime: str = "application/octet-stream",  # noqa: ARG001 — kept for API compat
     disabled: bool = False,
     help: str | None = None,
-    use_container_width: bool = True,
+    width: str = "stretch",
 ) -> None:
     """Save bytes directly to the user's Downloads folder.
 
@@ -497,7 +497,7 @@ def local_download_button(
         disabled=disabled,
         help=help,
         type="secondary",
-        use_container_width=use_container_width,
+        width=width,
     )
 
     if clicked:
@@ -1412,7 +1412,7 @@ def match_group_card(
                 return styles
 
             styled = compare_df.style.apply(_highlight_diffs, axis=0)
-            st.dataframe(styled, use_container_width=True)
+            st.dataframe(styled, width="stretch")
 
             if len(keep_indices) == n_rows:
                 st.info("Decision: Kept All")
@@ -1470,7 +1470,7 @@ def match_group_card(
                 editor_df,
                 column_config=col_config,
                 disabled=disabled_cols,
-                use_container_width=True,
+                width="stretch",
                 hide_index=True,
                 key=f"editor_{gid}",
             )
@@ -1514,7 +1514,7 @@ def match_group_card(
                     columns=["Keep"],
                 ).reset_index(drop=True)
                 st.markdown("**Surviving rows preview:**")
-                st.dataframe(preview, use_container_width=True, hide_index=True)
+                st.dataframe(preview, width="stretch", hide_index=True)
 
             # Confirm
             def _on_confirm(
@@ -1848,7 +1848,7 @@ def render_findings_panel(
                     _t("findings.open_tool", tool=name),
                     key=f"_findings_open_{tool_id}_{ns}",
                     type="primary",
-                    use_container_width=False,
+                    width="content",
                 ):
                     st.switch_page(page_slug)