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/app.d.ts

@@ -1,3 +1,6 @@
+/// <reference types="vite-plugin-pwa/client" />
+/// <reference types="vite-plugin-pwa/info" />
+
 // See https://svelte.dev/docs/kit/types#app.d.ts
 // for information about these interfaces
 declare global {