feat(06.2-02): frontend — is_shared badge fix + permission dropdown + View/Edit toggle

- DocumentCard.vue: fix Shared pill to read doc.is_shared (was doc.share_count > 0)
- ShareModal.vue: add permission select between handle input and submit button
- ShareModal.vue: replace static "view" span with View/Edit toggle group per share row
- ShareModal.vue: add handlePermissionChange with optimistic update + rollback on error
- documents.js: update shareDocument(docId, handle, permission='view') signature
- documents.js: add updateSharePermission(shareId, permission) action
- api/client.js: pass permission in createShare POST body
- api/client.js: add updateSharePermission PATCH helper

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

frontend/src/components/documents/DocumentCard.vue

@@ -28,7 +28,7 @@
         </div>
 
         <!-- Shared indicator pill -->
-        <div v-if="doc.share_count > 0" class="mt-2">
+        <div v-if="doc.is_shared" class="mt-2">
           <span class="bg-indigo-50 text-indigo-600 text-xs font-medium px-2 py-1 rounded-full">Shared</span>
         </div>
       </div>