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/alembic/versions/0d7cc7e3efb9_add_vote_deadline_to_communes.py

@@ -0,0 +1,30 @@
+"""add vote_deadline to communes
+
+Revision ID: 0d7cc7e3efb9
+Revises: 25f534648ea7
+Create Date: 2026-02-23 00:37:23.451137
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '0d7cc7e3efb9'
+down_revision: Union[str, None] = '25f534648ea7'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('communes', sa.Column('vote_deadline', sa.DateTime(), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('communes', 'vote_deadline')
+    # ### end Alembic commands ###