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this: a superseded call's own eventual timeout recognises it no longer owns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

src/web/public/styles.css

@@ -8577,11 +8577,37 @@ kbd {
 }
 
 .center-status-text {
+  flex: 1;
   pointer-events: auto;
   white-space: pre-wrap;
   word-break: break-word;
 }
 
+/* Error variant: the load didn't finish in time — nothing is "in progress" anymore (no
+   spinner), and since this one doesn't dismiss itself, it needs a close button the user
+   can actually click, so pointer-events is restored here too (see the wrapper's own
+   comment on why that's `none` by default). */
+.center-status-error {
+  border-color: rgba(239, 68, 68, 0.5);
+}
+
+.center-status-close {
+  flex-shrink: 0;
+  pointer-events: auto;
+  background: none;
+  border: none;
+  color: inherit;
+  opacity: 0.6;
+  font-size: 1.2rem;
+  line-height: 1;
+  padding: 0 0.15rem;
+  cursor: pointer;
+}
+
+.center-status-close:hover {
+  opacity: 1;
+}
+
 .toast-success { border-color: rgba(34, 197, 94, 0.4); }
 .toast-error { border-color: rgba(239, 68, 68, 0.4); }
 .toast-warning { border-color: rgba(234, 179, 8, 0.4); }