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>

src/lib/components/QrButton.svelte

@@ -0,0 +1,30 @@
+<script lang="ts">
+  import Icon from "@iconify/svelte";
+  import "$lib/iconify-offline";
+  import QrModal from "./QrModal.svelte";
+
+  let {
+    value,
+    label = "",
+    sublabel = "",
+    class: cls = "",
+  }: {
+    value: string;
+    label?: string;
+    sublabel?: string;
+    class?: string;
+  } = $props();
+
+  let open = $state(false);
+</script>
+
+<button
+  type="button"
+  title="QR-Code anzeigen"
+  onclick={() => (open = true)}
+  class={cls}
+>
+  <Icon icon="mdi:qrcode" class="size-[1em]" />
+</button>
+
+<QrModal value={open ? value : null} {label} {sublabel} onclose={() => (open = false)} />