fix: Bug - Two devices sharing same user says "Shared from your device"

Gate the "Shared from your device" label and the hidden download
affordance on whether this device actually holds the file bytes, not on
whether the current user uploaded it. uploaderPeerId is the user id, so
the old check claimed ownership on every device of the uploader,
blocking view/download on second devices that only synced metadata.

Also include attachment metadata in the account_sync chat-sync-batch so
sibling devices learn about synced attachments at all.

Co-authored-by: Cursor <cursoragent@cursor.com>

agents-docs/features/authentication.md

@@ -70,7 +70,7 @@ When the same account is logged in on multiple devices, account-owned data is ke
 | Data | Mechanism |
 |---|---|
 | Server chat messages (live) | `chat_message` signaling relay (connection-scoped broadcast) **plus** `account_sync` `chat-message` / `message-revision` to sibling devices |
-| Server chat messages (catch-up) | `account_sync` `chat-sync-batch` pushed when a sibling device comes online (`account_sync_peer_online`) |
+| Server chat messages (catch-up) | `account_sync` `chat-sync-batch` pushed when a sibling device comes online (`account_sync_peer_online`); each batch carries its messages' **attachment metadata** (`attachments` map, local paths stripped) so sibling devices learn about synced attachments — they are then requestable/downloadable but never marked "Shared from your device" unless the bytes are local |
 | Voice / typing | Existing `voice_state` / `user_typing` relays |
 | Saved servers (join/leave) | `account_sync` payload `saved-room-sync` / `saved-room-remove` |
 | Profile avatar + card text | `account_sync` `user-avatar-full` + `user-avatar-chunk` |