fix: auto-name uses session model + pane auto-remove on last tab close

Two independent UI/UX fixes:

- auto_name.ts: pass the session's own model as fallbackModel to
  taskModelCompletion, so chat rename uses whatever model is already
  loaded on llama-swap instead of forcing a swap to DEFAULT_MODEL
  (which times out at 10s when a different model is active).
- useWorkspacePanes.ts: when the last tab in a pane is closed and
  other panes exist, remove the pane entirely instead of leaving an
  orphaned empty panel.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/useWorkspacePanes.ts

@@ -391,6 +391,13 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
       const pane = next[paneIdx]!;
       const nextIds = pane.chatIds.filter((id) => id !== chatId);
       if (nextIds.length === 0) {
+        if (next.length > 1) {
+          // Last tab closed and other panes exist — remove the whole pane
+          // instead of leaving an orphaned empty panel.
+          const spliced = next.filter((_, i) => i !== paneIdx);
+          setActivePaneIdx((ai) => Math.min(ai, spliced.length - 1));
+          return spliced;
+        }
         next[paneIdx] = { ...pane, kind: 'empty', chatId: undefined, chatIds: [], activeChatIdx: -1 };
       } else {
         const nextActiveIdx = Math.min(pane.activeChatIdx, nextIds.length - 1);