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/pages/5_Column_Mapper.py

@@ -98,7 +98,7 @@ _has_result = st.session_state.get("colmap_result") is not None
 
 with st.expander(f"Preview: {uploaded.name}", expanded=not _has_result):
     st.caption(f"{len(df)} rows, {len(df.columns)} columns")
-    st.dataframe(df.head(10), use_container_width=True)
+    st.dataframe(df.head(10), width="stretch")
 st.divider()
 
 # ---------------------------------------------------------------------------
@@ -165,7 +165,7 @@ with st.expander("Options", expanded=not _has_result):
         })
         edited = st.data_editor(
             initial,
-            use_container_width=True,
+            width="stretch",
             num_rows="dynamic",
             column_config={
                 "name": st.column_config.TextColumn("Target name"),
@@ -276,7 +276,7 @@ with st.expander("Options", expanded=not _has_result):
         })
         rename_edited = st.data_editor(
             rename_initial,
-            use_container_width=True,
+            width="stretch",
             column_config={
                 "source": st.column_config.TextColumn("Source", disabled=True),
                 "target": st.column_config.TextColumn("Target"),
@@ -304,7 +304,7 @@ with st.expander("Options", expanded=not _has_result):
         })
         map_edited = st.data_editor(
             map_initial,
-            use_container_width=True,
+            width="stretch",
             column_config={
                 "source": st.column_config.TextColumn("Source", disabled=True),
                 "target": st.column_config.SelectboxColumn(
@@ -332,7 +332,7 @@ with st.expander("Options", expanded=not _has_result):
 
 st.divider()
 
-if st.button("Apply Column Mapping", type="primary", use_container_width=True):
+if st.button("Apply Column Mapping", type="primary", width="stretch"):
     with st.spinner("Mapping..."):
         try:
             result = map_columns(df, options)
@@ -400,10 +400,10 @@ if result.mapping:
             for s, t in result.mapping.items()
         ],
     )
-    st.dataframe(map_df, use_container_width=True, hide_index=True)
+    st.dataframe(map_df, width="stretch", hide_index=True)
 
 st.markdown("**Mapped preview (first 10 rows)**")
-st.dataframe(result.mapped_df.head(10), use_container_width=True)
+st.dataframe(result.mapped_df.head(10), width="stretch")
 
 # ---------------------------------------------------------------------------
 # Downloads