feat(themes): custom CSS themes per user (upload, paste, live)

- Theme manager in settings: upload a .css file or paste CSS, template
  download/copy/insert, edit and delete; custom themes show up in the
  gallery and the quick switchers
- Themes are injected live as <style id="mindsift-custom-theme">; the
  active one is cached so it applies on boot without flashing
- Backend: user_themes table and GET/POST /themes, DELETE /themes/{id}
  (per user, max 200 KB)
- Docs: theming, REST, database

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/api.ts

@@ -18,6 +18,7 @@ import {
   SpaceShare,
   SystemSettings,
   SharingMode,
+  UserTheme,
   Team,
   TaskList,
   TaskListShare,
@@ -1009,6 +1010,23 @@ export const api = {
     });
   },
 
+  // --- CUSTOM THEMES ---
+  async getThemes(): Promise<UserTheme[]> {
+    return ensureArray(await request<UserTheme[]>('/api/v1/themes'));
+  },
+
+  // Creates a theme, or updates it when `id` is given
+  async saveTheme(theme: { id?: string; name: string; css: string }): Promise<UserTheme> {
+    return request<UserTheme>('/api/v1/themes', {
+      method: 'POST',
+      body: JSON.stringify(theme),
+    });
+  },
+
+  async deleteTheme(id: string): Promise<void> {
+    await request(`/api/v1/themes/${id}`, { method: 'DELETE' });
+  },
+
   // --- API KEYS, WEBHOOKS & EXPORT ---
   async getAPIKeys(): Promise<APIKey[]> {
     const res = await request<APIKey[]>('/api/v1/keys');