feat: complete user config management

- Add theme support with dark/light/system modes
- Add useTheme hook for applying user config theme
- Update router to use SettingsPage
- Update app-shell to apply theme on load
- Add CSS variables for dark theme
- Fix mypy errors in user_config.py
- Quality gates pass: ruff, mypy, typecheck, lint, build

apps/api/alembic/versions/0003_user_configs.py

@@ -0,0 +1,35 @@
+"""add user_configs table
+
+Revision ID: 0003
+Revises: 0002
+Create Date: 2025-05-18
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision: str = '0003'
+down_revision: Union[str, None] = '0002'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        'user_configs',
+        sa.Column('id', sa.UUID(), nullable=False),
+        sa.Column('user_id', sa.UUID(), nullable=False),
+        sa.Column('config', sa.JSON(), nullable=False),
+        sa.Column('created_at', sa.DateTime(timezone=True), server_default=sa.text('now()'), nullable=False),
+        sa.Column('updated_at', sa.DateTime(timezone=True), server_default=sa.text('now()'), nullable=False),
+        sa.ForeignKeyConstraint(['user_id'], ['users.id'], ),
+        sa.PrimaryKeyConstraint('id'),
+        sa.UniqueConstraint('user_id')
+    )
+
+
+def downgrade() -> None:
+    op.drop_table('user_configs')