Restructure citizen vote page + add vote deadline

Reorganize the citizen page (/commune/[slug]) for voters: full-width
interactive chart with "Population" zoom by default, separate auth and
vote sections, countdown timer for vote deadline.

Backend: add vote_deadline column to communes with Alembic migration.
Admin: add deadline configuration card with datetime-local input.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/routers/communes.py

@@ -67,6 +67,8 @@ async def update_commune(
         commune.description = data.description
     if data.is_active is not None:
         commune.is_active = data.is_active
+    if data.vote_deadline is not None:
+        commune.vote_deadline = data.vote_deadline
 
     await db.commit()
     await db.refresh(commune)