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/6_Outlier_Detector.py

@@ -72,7 +72,7 @@ if uploaded is not None:
             df = pd.read_csv(uploaded)
         st.subheader(f"Preview: {uploaded.name}")
         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")
     except Exception as e:
         from src.core.errors import format_for_user
         st.error(
@@ -95,5 +95,5 @@ st.subheader("Handling")
 st.selectbox("Action", ["Flag only (add column)", "Remove outlier rows", "Cap / Winsorize to bounds"], disabled=True)
 
 st.divider()
-st.button("Detect Outliers", type="primary", use_container_width=True, disabled=True)
+st.button("Detect Outliers", type="primary", width="stretch", disabled=True)