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/_home.py

@@ -174,7 +174,7 @@ def _home_page() -> None:
                 key=f"_home_remove_{digest}",
                 help=f"Remove {name}",
                 type="secondary",
-                use_container_width=True,
+                width="stretch",
             ):
                 to_remove = name
 
@@ -240,14 +240,14 @@ def _home_page() -> None:
             type="primary",
             key="home_run_analysis",
             disabled=not pending,
-            use_container_width=True,
+            width="stretch",
         )
     with col_clear:
         clear_clicked = st.button(
             t("upload.clear_results"),
             key="home_clear_results",
             disabled=not findings_by_file,
-            use_container_width=True,
+            width="stretch",
         )
 
     if clear_clicked: