feat(custom-model): live countdown on the loading banner; timeout is now an error

The loading banner now shows a live countdown against its own timeout
(updated every poll, so every second by default) instead of a static
"this can take a while" — e.g. "Loading qwen3.8-27b (16.4 GB, typically
~1-3 min) on llama-swap - 47s remaining".

If the countdown reaches zero and the model still isn't ready, this is now
treated as a real failure rather than a "keep waiting" shrug:
- The banner turns into a sticky error (_showCenterStatus gains a `type`
  option - 'error' drops the spinner and adds a close button, since nothing
  is "in progress" anymore and a sticky message needs a way to dismiss it),
  naming the llama-swap server's own logs as where to look for detail.
- The session that load was for is closed automatically (closeSession) -
  requested explicitly: a console left open and pointed at a model that
  never finished loading is worse than no console at all. Both apply paths
  now thread the new session's id through to _watchLlamaSwapLoading for
  this (new required 3rd parameter, after endpointId/modelId).

_watchLlamaSwapGeneration's existing stale-call guard extends naturally to
this: a superseded call's own eventual timeout recognises it no longer owns
the banner and neither shows the error nor closes a session that may by
then belong to a different, newer launch.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

test/custom-model-one-shot-launch.test.ts

@@ -108,17 +108,17 @@ describe('_runCustomModelEntryOneShot', () => {
     expect(app._pendingCustomModelForLaunch).toBeUndefined();
   });
 
-  it('starts the loading watcher when the launch reports modelSwapInProgress', async () => {
+  it('starts the loading watcher when the launch reports modelSwapInProgress, passing the new session id', async () => {
     const { app } = bootApp();
     app.run = async () => {
-      app._lastCustomModelLaunchResult = { modelSwapInProgress: true };
+      app._lastCustomModelLaunchResult = { modelSwapInProgress: true, sessionId: 'new-session' };
     };
     let watched: unknown[] | null = null;
     app._watchLlamaSwapLoading = async (...args: unknown[]) => {
       watched = args;
     };
     await app._runCustomModelEntryOneShot('codex', 'llama-box', 'qwen3');
-    expect(watched).toEqual(['llama-box', 'qwen3']);
+    expect(watched).toEqual(['llama-box', 'qwen3', 'new-session']);
   });
 
   it('never starts the watcher when no swap was needed', async () => {