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/7_Multi_File_Merger.py

@@ -74,7 +74,7 @@ if uploaded_files:
                 df = pd.read_csv(f)
             st.subheader(f"Preview: {f.name}")
             st.caption(f"{len(df)} rows, {len(df.columns)} columns — Columns: {', '.join(df.columns[:10])}{'...' if len(df.columns) > 10 else ''}")
-            st.dataframe(df.head(5), use_container_width=True)
+            st.dataframe(df.head(5), width="stretch")
         except Exception as e:
             from src.core.errors import format_for_user
             st.error(
@@ -93,5 +93,5 @@ st.selectbox("Mismatched columns", ["Fill with null", "Drop non-shared columns",
 st.checkbox("Add source filename column", value=True, disabled=True)
 
 st.divider()
-st.button("Merge Files", type="primary", use_container_width=True, disabled=True)
+st.button("Merge Files", type="primary", width="stretch", disabled=True)