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>

frontend/src/components/Sidebar.tsx

@@ -15,6 +15,7 @@ import {
   Folder,
   Users,
   UsersRound,
+  Palette,
 } from "lucide-react";
 import { Button } from "@/components/ui/button";
 import ThemeToggle from "@/components/ThemeToggle";
@@ -265,6 +266,13 @@ export default function Sidebar() {
                   <UsersRound className="h-4 w-4 shrink-0" />
                   <span className="whitespace-nowrap">Groups</span>
                 </NavLink>
+                <NavLink
+                  to="/admin/appearance"
+                  className={({ isActive }) => subItemClass(isActive)}
+                >
+                  <Palette className="h-4 w-4 shrink-0" />
+                  <span className="whitespace-nowrap">Appearance</span>
+                </NavLink>
               </div>
             )}
           </div>