feat: QR-Code, vCard-Export, Org-Gruppenansicht im Adressbuch

- vcard.ts: shared vCard-Generator (downloadVCard, downloadVCardAll, buildVCardString)
- QrModal.svelte + QrButton.svelte: wiederverwendbare QR-Komponenten mit PNG-Download
- AdressbuchBlock: Org-Gruppenansicht (Toggle), QR + vCard pro Kontakt,
  Buchstaben-Trennzeilen, Such-Highlight, "Alle exportieren", Print-Styles
- ContactCard: vCard-Download-Button
- PostActions: QR-Button für Seiten-URL
- qrcode npm-Paket hinzugefügt

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -26,6 +26,7 @@
     "@sveltejs/vite-plugin-svelte": "^5.0.3",
     "@tailwindcss/vite": "^4.1.18",
     "@types/node": "^22.10.0",
+    "@types/qrcode": "^1.5.6",
     "@types/sanitize-html": "^2.13.0",
     "@vitest/coverage-v8": "^2.1.9",
     "openapi-typescript": "^7.4.0",
@@ -47,6 +48,7 @@
     "class-variance-authority": "^0.7.1",
     "leaflet": "^1.9.4",
     "marked": "^17.0.2",
+    "qrcode": "^1.5.4",
     "sanitize-html": "^2.13.1",
     "sharp": "^0.34.5"
   },