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

.gitignore

@@ -1,35 +1,23 @@
-# See https://help.github.com/articles/ignoring-files/ for more about ignoring files.
-
 # dependencies
 /node_modules
-/.pnp
-.pnp.js
-.yarn/install-state.gz
-
-# testing
-/coverage
 
 # next.js
 /.next/
 /out/
-
-# production
 /build
 
-# misc
-.DS_Store
-*.pem
+# testing
+/coverage
+/playwright-report
+/test-results
 
-# debug
-npm-debug.log*
-yarn-debug.log*
-yarn-error.log*
-
-# local env files
-.env*.local
-
-# vercel
-.vercel
+# local env files — only .env.example is committed
+.env
+.env.local
+.env.*.local
+.env.development
+.env.production
+.env.test
 
 # claude code personal settings
 .claude/settings.local.json
@@ -37,3 +25,8 @@ yarn-error.log*
 # typescript
 *.tsbuildinfo
 next-env.d.ts
+
+# misc
+.DS_Store
+*.pem
+npm-debug.log*