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/schemas/schemas.py

@@ -35,6 +35,7 @@ class CommuneUpdate(BaseModel):
     name: str | None = None
     description: str | None = None
     is_active: bool | None = None
+    vote_deadline: datetime | None = None
 
 
 class CommuneOut(BaseModel):
@@ -44,6 +45,7 @@ class CommuneOut(BaseModel):
     description: str
     is_active: bool
     created_at: datetime
+    vote_deadline: datetime | None = None
 
     model_config = {"from_attributes": True}