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/models/models.py

@@ -29,6 +29,7 @@ class Commune(Base):
     description = Column(Text, default="")
     is_active = Column(Boolean, default=True)
     created_at = Column(DateTime, default=datetime.utcnow)
+    vote_deadline = Column(DateTime, nullable=True)
 
     tariff_params = relationship("TariffParams", back_populates="commune", uselist=False)
     households = relationship("Household", back_populates="commune")