fix(06.2): resolve four sharing UX issues found in re-test UAT

- AccountView: remove hardcoded @ prefix so handle matches what share dialog expects
- documents store: set is_shared=true optimistically after successful share so badge shows without refetch
- GET /api/documents/{id}: allow recipients of an active share to view the document (was returning 404 for non-owners)
- ShareModal: move Share button to its own full-width row so it no longer overflows the input area

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

frontend/src/stores/documents.js

@@ -157,7 +157,10 @@ export const useDocumentsStore = defineStore('documents', () => {
   }
 
   async function shareDocument(docId, recipientHandle, permission = 'view') {
-    try { return await api.createShare(docId, recipientHandle, permission) } catch (e) { throw e }
+    const result = await api.createShare(docId, recipientHandle, permission)
+    const doc = documents.value.find(d => d.id === docId)
+    if (doc) doc.is_shared = true
+    return result
   }
 
   async function revokeShare(shareId) {