Add theming system: custom palettes, per-user colour mode, admin appearance page

- 4 built-in themes (Default, Pastel, High Contrast, Ocean Blue) seeded as
  JSON files in /config/themes/ on startup; custom themes can be created,
  edited, and deleted via the new admin Appearance page
- All theme tokens applied via JS inline CSS properties (no hardcoded CSS blocks)
- New `color_mode` column on users table (migration dd6ad2f2c211); users can
  override the admin-set global default in Settings
- Backend: GET/PATCH /settings/appearance, full CRUD on /settings/themes
- Frontend: AdminAppearancePage with theme grid + colour pickers, SettingsPage
  replaces placeholder with mode selector, useTheme rewritten to fetch from API

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

backend/alembic/versions/dd6ad2f2c211_add_color_mode_to_users.py

@@ -0,0 +1,25 @@
+"""add_color_mode_to_users
+
+Revision ID: dd6ad2f2c211
+Revises: c7e8f9a0b1d2
+Create Date: 2026-04-17 23:42:58.222958
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision: str = 'dd6ad2f2c211'
+down_revision: Union[str, None] = 'c7e8f9a0b1d2'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('users', sa.Column('color_mode', sa.String(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column('users', 'color_mode')