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

Dockerfile

@@ -0,0 +1,41 @@
+FROM node:22-alpine AS base
+
+# --- deps: install production-resolvable dependencies once -------------------
+FROM base AS deps
+WORKDIR /app
+COPY package.json package-lock.json ./
+RUN npm ci
+
+# --- builder: compile the Next.js standalone bundle --------------------------
+FROM base AS builder
+WORKDIR /app
+COPY --from=deps /app/node_modules ./node_modules
+COPY . .
+ENV NEXT_TELEMETRY_DISABLED=1
+RUN npm run build
+
+# --- runner: minimal runtime image -------------------------------------------
+FROM base AS runner
+WORKDIR /app
+
+ENV NODE_ENV=production
+ENV NEXT_TELEMETRY_DISABLED=1
+ENV PORT=3000
+ENV HOSTNAME=0.0.0.0
+
+RUN addgroup --system --gid 1001 nodejs \
+  && adduser --system --uid 1001 nextjs
+
+COPY --from=builder /app/public ./public
+# standalone already contains the pruned node_modules and server.js
+COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
+COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
+
+USER nextjs
+EXPOSE 3000
+
+# Used by compose's healthcheck and by scripts/update.sh for rollback detection
+HEALTHCHECK --interval=30s --timeout=5s --start-period=20s --retries=3 \
+  CMD node -e "fetch('http://127.0.0.1:3000/api/health').then(r=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"
+
+CMD ["node", "server.js"]