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/3_Format_Standardizer.py

@@ -107,7 +107,7 @@ except Exception as e:
 _has_result = st.session_state.get("fmtstd_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()
 
 
@@ -478,7 +478,7 @@ with st.expander("Options", expanded=not _has_result):
             edited = st.data_editor(
                 starter,
                 num_rows="dynamic",
-                use_container_width=True,
+                width="stretch",
                 column_config={
                     "abbreviation": st.column_config.TextColumn(
                         "Short form",
@@ -530,7 +530,7 @@ run_disabled = not column_types
 if st.button(
     "Standardize Formats",
     type="primary",
-    use_container_width=True,
+    width="stretch",
     disabled=run_disabled,
 ):
     with st.spinner("Standardizing..."):
@@ -592,16 +592,16 @@ if result.cells_changed:
     st.markdown("**Changes by column**")
     st.dataframe(
         counts.rename("cells_changed").to_frame(),
-        use_container_width=True,
+        width="stretch",
     )
 
     st.markdown("**Examples (first 25 changes)**")
     examples = result.changes.head(25).copy()
     examples["row"] = examples["row"] + 1
-    st.dataframe(examples, use_container_width=True, hide_index=True)
+    st.dataframe(examples, width="stretch", hide_index=True)
 
 st.markdown("**Standardized preview (first 10 rows)**")
-st.dataframe(result.standardized_df.head(10), use_container_width=True)
+st.dataframe(result.standardized_df.head(10), width="stretch")
 
 
 # ---------------------------------------------------------------------------