Add Stundenplan frontend scaffolding in studio-v2

Phase 3 — initial UI for the timetable scheduler:
  - app/stundenplan/page.tsx with tab navigation (Klassen / Lehrer /
    Faecher / Raeume / Zeitraster / Stundentafel / Lehrauftraege /
    Regeln) and a dev-mode JWT entry to authenticate against
    school-service until full auth is wired up.
  - app/stundenplan/_components/KlassenManager.tsx as the working
    prototype for one entity (list / create / delete). Pattern can be
    copied for the other 6 stammdaten + 15 constraint editors.
  - lib/stundenplan/api.ts exposing typed clients for all 22 endpoints
    (7 stammdaten + 15 constraint tables). Constraints use a factory
    to keep the file tight.
  - app/api/school/[...path]/route.ts proxies the browser through
    Next.js to school-service so HTTPS studio-v2 can reach the plain
    HTTP backend.
  - Sidebar.tsx gains a Stundenplan entry with 26-language labels.
  - docker-compose.yml exposes SCHOOL_SERVICE_URL to studio-v2 and
    declares the school-service dependency.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -109,8 +109,10 @@ services:
     environment:
       NODE_ENV: production
       BACKEND_URL: http://backend-lehrer:8001
+      SCHOOL_SERVICE_URL: http://school-service:8084
     depends_on:
       - backend-lehrer
+      - school-service
     restart: unless-stopped
     networks:
       - breakpilot-network