fix(threads): enable slash commands in the thread composer

The thread composer already showed the /command autocomplete (RoomInput.tsx:954
was never gated), but the interpreter was disabled (:523), so /me, /shrug,
/invite, etc. sent literally in threads - a confusing inconsistency and the
other half of the threads "v1" limitation.

Remove the thread gate: content-transform commands (/me, /notice, /shrug,
/tableflip, /unflip) flow into the normal send path, which already routes to the
thread via threadRootId; the rest are room-level actions. No command sends a
mis-routed timeline message (verified against useCommands). Scheduling stays
disabled in threads for now.

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

src/app/features/room/RoomInput.tsx

@@ -518,9 +518,12 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
     const submit = useCallback(() => {
       uploadBoardHandlers.current?.handleSend();
 
-      // Slash-command interpretation is disabled in thread mode (v1): "/foo"
-      // sends literally rather than being parsed as a command.
-      const commandName = threadRootId ? undefined : getBeginCommand(editor);
+      // Slash commands work in threads too: content-transform commands (/me,
+      // /notice, /shrug, /tableflip, /unflip) flow into the normal send below,
+      // which routes to the thread via `threadRootId`; the rest (/invite, /kick,
+      // …) are room-level actions. This also matches the command autocomplete,
+      // which is already shown in the thread composer.
+      const commandName = getBeginCommand(editor);
       let plainText = toPlainText(editor.children, isMarkdown).trim();
       let customHtml = trimCustomHtml(
         toMatrixCustomHTML(editor.children, {