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/schemas/user.py

@@ -71,6 +71,7 @@ class UserOut(BaseModel):
     # validation_alias reads is_superuser from the ORM object; the JSON key
     # in the response is the field name "is_admin" (not the alias).
     is_admin: bool = Field(validation_alias="is_superuser", default=False)
+    color_mode: str | None = None
 
     model_config = {"from_attributes": True, "populate_by_name": True}
 
@@ -104,6 +105,17 @@ class DashboardPrefsOut(BaseModel):
     app_ids: list[str]
 
 
+class ColorModeUpdate(BaseModel):
+    color_mode: str
+
+    @field_validator("color_mode")
+    @classmethod
+    def validate_mode(cls, v: str) -> str:
+        if v not in ("light", "dark", "system"):
+            raise ValueError("color_mode must be 'light', 'dark', or 'system'")
+        return v
+
+
 class DashboardPrefsUpdate(BaseModel):
     app_ids: list[str] = Field(default_factory=list)