feat: image/file sharing in chat with drive grouping by contact

- Attach button in chat sends encrypted files via conversation key
- Images render inline; other files show a download button
- Shared files stored on server with conversation_id for access control
- Drive groups files by conversation partner name (sender view)

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

backend/src/routes/files.ts

@@ -57,8 +57,15 @@ router.post('/upload', upload.single('file'), async (req: AuthRequest, res) => {
 
 router.get('/files', async (req: AuthRequest, res) => {
   const result = await db.query(
-    `SELECT id, encrypted_name, mime_type, size_bytes, encrypted_file_key, uploaded_at
-     FROM files WHERE owner_id = $1 AND deleted_at IS NULL ORDER BY uploaded_at DESC`,
+    `SELECT f.id, f.encrypted_name, f.mime_type, f.size_bytes, f.encrypted_file_key,
+            f.uploaded_at, f.conversation_id,
+            u.username AS shared_with_username
+     FROM files f
+     LEFT JOIN conversation_members cm
+       ON cm.conversation_id = f.conversation_id AND cm.user_id != $1
+     LEFT JOIN users u ON u.id = cm.user_id
+     WHERE f.owner_id = $1 AND f.deleted_at IS NULL
+     ORDER BY f.uploaded_at DESC`,
     [req.userId]
   );
   res.json(result.rows);