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>

Doku/api/rest.md

@@ -30,6 +30,8 @@ Basis: `/api/v1`. Antworten sind JSON; Fehler haben die Form `{"error": "Text"}`
 | GET | `/stats` | Zähler fürs Dashboard |
 | GET | `/search?q=` | Suche in eigenen Gedanken und Notizen |
 | GET | `/export` | Datenexport als JSON |
+| GET/POST | `/themes` | Eigene CSS-Themes; anlegen oder mit `id` ändern (`name`, `css`, max. 200 KB) |
+| DELETE | `/themes/{id}` | Eigenes Theme löschen |
 | GET/POST/DELETE | `/keys`, `/keys/{id}` | API-Keys verwalten (Klartext nur bei Erstellung) |
 
 ## Gedanken, Triage, Aufgaben