feat: Share-Link Typ 'Nur Upload' (upload_only)

Dritter Link-Typ neben read und write:
- upload_only: Nur Dateien hochladen, kein Download, kein Ordnerinhalt
  sichtbar, Ordnername wird nicht angezeigt

Backend-Absicherung:
- GET /share/<token>/download gibt 403 bei upload_only
- POST /share/<token>/upload erlaubt upload_only + write
- GET /share/<token>/info gibt download_allowed zurueck

Frontend Share-Dialog:
- Drei Optionen: Nur Lesen / Lesen+Hochladen / Nur Upload
- Bestehende Links zeigen Typ an

Frontend ShareView:
- upload_only: Zeigt nur Upload-Zone, kein Dateiname, kein Download
- Hinweistext 'Dieser Link erlaubt nur das Hochladen von Dateien'

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/FilesView.vue

@@ -184,7 +184,7 @@
             <div class="link-info">
               <code>{{ currentOrigin }}/share/{{ link.token }}</code>
               <small>
-                {{ link.permission === 'write' ? 'Lesen+Schreiben' : 'Nur Lesen' }}
+                {{ {read: 'Nur Lesen', write: 'Lesen+Schreiben', upload_only: 'Nur Upload'}[link.permission] || link.permission }}
                 | {{ link.download_count }} Downloads
                 <template v-if="link.expires_at"> | Bis {{ formatDate(link.expires_at) }}</template>
                 <template v-if="link.has_password"> | Passwortgeschuetzt</template>
@@ -255,7 +255,11 @@ const selectedShareUser = ref(null)
 const shareUserPermission = ref('read')
 const userSearchResults = ref([])
 const userPermOptions = [{ label: 'Lesen', value: 'read' }, { label: 'Schreiben', value: 'write' }, { label: 'Admin', value: 'admin' }]
-const linkPermOptions = [{ label: 'Nur Lesen (Download)', value: 'read' }, { label: 'Lesen + Hochladen (nur Ordner)', value: 'write' }]
+const linkPermOptions = [
+  { label: 'Nur Lesen (Download)', value: 'read' },
+  { label: 'Lesen + Hochladen (nur Ordner)', value: 'write' },
+  { label: 'Nur Upload (Ordner, kein Einblick)', value: 'upload_only' },
+]
 const shareLinkPermission = ref('read')
 const currentOrigin = window.location.origin
 const shareLoading = ref(false)