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/app/models/user.py

@@ -23,6 +23,8 @@ class User(Base):
     is_superuser: Mapped[bool] = mapped_column(Boolean, default=False)
     # List of service IDs pinned to the user's home dashboard.
     dashboard_app_ids: Mapped[list] = mapped_column(JSON, nullable=False, default=list)
+    # User's preferred colour mode: "light", "dark", "system", or None (use admin default).
+    color_mode: Mapped[str | None] = mapped_column(String, nullable=True, default=None)
 
     profile: Mapped["Profile"] = relationship(
         "Profile", back_populates="user", uselist=False, cascade="all, delete-orphan"