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/9_Pipeline_Runner.py

@@ -99,7 +99,7 @@ _has_result = st.session_state.get("pipeline_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()
 
@@ -170,7 +170,7 @@ with st.expander("Options", expanded=not _has_result):
     )
     edited = st.data_editor(
         st.session_state["pipeline_rows"],
-        use_container_width=True,
+        width="stretch",
         num_rows="dynamic",
         column_config={
             "tool": st.column_config.SelectboxColumn(
@@ -246,7 +246,7 @@ run_disabled = current_pipeline is None or not current_pipeline.steps
 if st.button(
     "Run Pipeline",
     type="primary",
-    use_container_width=True,
+    width="stretch",
     disabled=run_disabled,
 ):
     progress = st.progress(0.0, text="Starting...")
@@ -343,10 +343,10 @@ step_df = pd.DataFrame([
     }
     for sr in result.step_results
 ])
-st.dataframe(step_df, use_container_width=True, hide_index=True)
+st.dataframe(step_df, width="stretch", hide_index=True)
 
 st.markdown("**Output preview (first 10 rows)**")
-st.dataframe(result.final_df.head(10), use_container_width=True)
+st.dataframe(result.final_df.head(10), width="stretch")
 
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