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

docs/PRD.md

@@ -1,29 +1,31 @@
 # Product Requirements Document
 
 ## Vision
-_Describe what you are building and why._
+_Was wird gebaut und warum? Zwei bis drei Sätze._
 
-## Target Users
-_Who will use this product? Describe their needs and pain points._
+## Zielgruppe
+_Wer nutzt das Produkt? Welchen konkreten Schmerz hat diese Gruppe heute?_
 
-## Core Features (Roadmap)
+## Roadmap
 
-| Priority | Feature | Status |
-|----------|---------|--------|
-| P0 (MVP) | _Feature 1_ | Planned |
-| P0 (MVP) | _Feature 2_ | Planned |
-| P1 | _Feature 3_ | Planned |
-| P2 | _Feature 4_ | Planned |
+| Priorität | Feature | Status |
+|-----------|---------|--------|
+| P0 (MVP) | _Feature 1_ | Roadmap |
+| P0 (MVP) | _Feature 2_ | Roadmap |
+| P1 | _Feature 3_ | Roadmap |
+| P2 | _Feature 4_ | Roadmap |
 
-## Success Metrics
-_How will you measure success? (e.g., user signups, retention, task completion rate)_
+## Erfolgsmetriken
+_Woran wird gemessen, ob das Produkt funktioniert?_
 
-## Constraints
-_Budget, timeline, technical limitations, team size._
+## Rahmenbedingungen
+_Zeit, Budget, Teamgröße, technische Grenzen. Auch: Braucht das Projekt dauerhafte
+Datenspeicherung? Braucht es Nutzerkonten?_
 
-## Non-Goals
-_What are you explicitly NOT building in this version?_
+## Nicht-Ziele
+_Was in dieser Version bewusst nicht gebaut wird._
 
 ---
 
-Use `/requirements` to create detailed feature specifications for each item in the roadmap above.
+Dieses Dokument füllt `/init` im Interview aus. Danach entstehen mit `/write-spec`
+die einzelnen Feature-Specs.