pwa / offline mode

- @vite-pwa/sveltekit: workbox precache (incl. pdfjs worker), generated manifest, prompt-style updates via PwaUpdatePrompt toast
- prerendered routes for offline navigation, PWA icons, theme-color + apple-touch-icon
- pwa offline/update e2e suite (pnpm test:pwa, playwright.pwa.config.ts)
- datenschutz: offline/install section; prettier cleanup of shadcn components

src/routes/+layout.ts

@@ -0,0 +1,5 @@
+// Every route is rendered from static markup and processes data purely in the browser, so all
+// pages are prerendered. That is what makes the PWA work offline: `@vite-pwa/sveltekit` precaches
+// `prerendered/**/*.{html,json}` (see vite.config.ts), so the service worker can answer document
+// navigations without a server.
+export const prerender = true;