refactor: rebuild template — Tailwind v4, Docker self-hosting, passkey auth

Toolchain fixes (all previously broken):
- npm run lint: next lint was removed in Next 16 -> eslint with flat config
- .eslintrc.json was ignored by ESLint 9 -> eslint.config.mjs
- npm test / test:e2e exited 1 with no tests -> passWithNoTests
- Vitest scanned tests/ and would have run Playwright specs -> scoped to src/
- Playwright targeted Mobile Safari while docs installed only chromium
- reuseExistingServer silently tested a foreign server on :3000 -> E2E_PORT
- 11 high-severity vulns -> npm audit fix
- deny rule Read(**/.env*) also blocked .env.example -> precise patterns

Stack changes:
- Tailwind v3 -> v4 (CSS-first, no tailwind.config.ts, tw-animate-css)
- shadcn regenerated for v4; 33 components -> 6 core, bundled radix-ui
- Supabase removed entirely; data layer is now chosen per project
- Vercel removed; Docker Compose self-hosting with scripts/update.sh
- Better Auth + passkey plugin as the binding auth standard
- ThemeProvider and Sonner mounted; dark mode and toasts now actually work

Workflow system:
- Sub-agents wired: /frontend, /backend, /qa delegate to .claude/agents
- Checklists folded into their SKILL.md; one file per skill
- Rules rewritten: DB-agnostic backend, passkey security, v4 frontend
- Spec language settled as German, skill instructions English

features/INDEX.md

@@ -1,21 +1,24 @@
 # Feature Index
 
-> Central tracking for all features. Updated by skills automatically.
+> Zentrales Tracking aller Features. Wird von den Skills automatisch gepflegt.
 
-## Status Legend
-- **Roadmap** - `/init` done, feature identified in feature map, no spec file yet
-- **Planned** - `/write-spec` done, full spec written, architecture not yet designed
-- **Architected** - `/architecture` done, tech design approved, ready to build
-- **In Progress** - `/frontend` or `/backend` active or completed, not yet in QA
-- **In Review** - `/qa` active, testing in progress
-- **Approved** - `/qa` passed, no critical/high bugs, ready to deploy
-- **Deployed** - `/deploy` done, live in production
+## Status
+
+`Roadmap` → `Planned` → `Architected` → `In Progress` → `In Review` → `Approved` → `Deployed`
+
+| Status | Bedeutung |
+|--------|-----------|
+| Roadmap | In `/init` identifiziert, noch keine Spec-Datei |
+| Planned | `/write-spec` fertig — Spec geschrieben |
+| Architected | `/architecture` fertig — technisches Design steht |
+| In Progress | `/frontend` oder `/backend` läuft oder ist fertig |
+| In Review | `/qa` läuft |
+| Approved | `/qa` bestanden, keine Critical/High-Bugs |
+| Deployed | `/deploy` fertig, live |
 
 ## Features
 
-| ID | Feature | Status | Spec | Created |
-|----|---------|--------|------|---------|
+| ID | Feature | Priorität | Abhängigkeiten | Status | Spec |
+|----|---------|-----------|----------------|--------|------|
 
-<!-- Add features above this line -->
-
-## Next Available ID: PROJ-1
+## Nächste freie ID: PROJ-1