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>

backend/api/documents.py

@@ -542,9 +542,20 @@ async def get_document(
         raise HTTPException(404, "Document not found")
 
     doc = await session.get(Document, uid)
-    if doc is None or doc.user_id != current_user.id:
+    if doc is None:
         raise HTTPException(404, "Document not found")
 
+    if doc.user_id != current_user.id:
+        # Allow recipients of an active share to view the document
+        share_result = await session.execute(
+            select(Share).where(
+                Share.document_id == uid,
+                Share.recipient_id == current_user.id,
+            )
+        )
+        if share_result.scalar_one_or_none() is None:
+            raise HTTPException(404, "Document not found")
+
     meta = await storage.get_metadata(session, doc_id)
     if meta is None:
         raise HTTPException(404, "Document not found")