feat(threads): up-arrow edits your last thread reply (+ fix cross-fire)

The thread composer reused RoomInput's hardcoded editableName="RoomInput", so
the main timeline's global up-arrow "edit last message" handler fired while
focused in a thread composer and targeted the MAIN timeline's last message
(wrong), and there was no up-arrow edit for the thread itself.

- Make editableName a RoomInput prop (default "RoomInput"); the thread composer
  passes "ThreadInput", so the two up-arrow handlers never cross-fire.
- Add an up-arrow-edit handler to ThreadTimeline (parity with RoomTimeline):
  empty thread composer + Up -> edit the latest editable reply in that thread,
  using thread.liveTimeline + canEditEvent + setEditId.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/room/thread/ThreadPanel.tsx

@@ -196,6 +196,7 @@ export function ThreadPanel({ room, threadId, requestClose }: ThreadPanelProps)
               roomId={room.roomId}
               threadRootId={threadId}
               editor={editor}
+              editableName="ThreadInput"
               fileDropContainerRef={fileDropContainerRef}
             />
           </Box>