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/RoomInput.tsx

@@ -151,9 +151,13 @@ interface RoomInputProps {
   roomId: string;
   room: Room;
   threadRootId?: string;
+  // Identifies this composer to global key handlers (e.g. the up-arrow "edit last
+  // message" handler). Threads pass a distinct name so the main timeline's handler
+  // doesn't fire for the thread composer, and vice-versa.
+  editableName?: string;
 }
 export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
-  ({ editor, fileDropContainerRef, roomId, room, threadRootId }, ref) => {
+  ({ editor, fileDropContainerRef, roomId, room, threadRootId, editableName = 'RoomInput' }, ref) => {
     const mx = useMatrixClient();
     const useAuthentication = useMediaAuthentication();
     const [enterForNewline] = useSetting(settingsAtom, 'enterForNewline');
@@ -981,7 +985,7 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
         )}
         <ScheduledMessagesTray roomId={roomId} />
         <CustomEditor
-          editableName="RoomInput"
+          editableName={editableName}
           editor={editor}
           placeholder="Send a message..."
           onKeyDown={handleKeyDown}