feat(custom-model): remove loading-banner countdown, add manual Cancel

Replaces the size-scaled expected-time estimate + matching auto-timeout
with a generic hardware/model-size disclaimer and a user-driven Cancel
button, per explicit request. Real load time depends on hardware this
feature has no way to know (VRAM, storage speed, GPU contention), so
the old estimate/timeout was a guess dressed up as a fact — worse, one
that could kill a genuinely slow load partway through on slower
hardware.

- _watchLlamaSwapLoading (session-ui.js): dropped maxWaitMs/deadline
  entirely — polls indefinitely until ready or cancelled, no automatic
  give-up. Message is now "Loading <model> (<size>) on <endpoint> —
  this can take a while depending on your hardware and the model
  size.", with the real llama.cpp log line still on its own second
  line. Removed _MODEL_LOAD_TIME_MATRIX/_estimateModelLoad/
  _formatRemaining (dead code once the countdown is gone) —
  _lookupModelSizeGB is kept, the GB figure still shows.
- _showCenterStatus (panels-ui.js) gains opts.onCancel: renders a real
  "Cancel" button (distinct from the error-type "×" close button,
  since Cancel has a real consequence) that calls it on click. Caller
  owns what cancelling actually means, same split as the swap-confirm
  modal's promise-resolving buttons.
- Cancelling dismisses the banner, shows an info toast (not an error —
  this was deliberate), and closes the session, mirroring what the old
  timeout used to do automatically but now on the user's own call.
- New .center-status-cancel CSS (bordered pill button, distinct from
  the plain "×" close glyph).

Test changes: removed the now-invalid timeout-auto-close/estimate
tests, added cancel-flow tests (dismiss/toast-type/session-close,
never-closes-with-no-sessionId, unbounded-polling), and real-DOM tests
for the new Cancel button (bootAppWithRealCenterStatus, evaluating
panels-ui.js instead of stubbing _showCenterStatus, since this button
is worth verifying for real rather than just through the stub every
other test in the file uses). Typecheck/lint/frontend-syntax clean;
full suite shows no new regressions.

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

src/web/public/styles.css

@@ -8636,6 +8636,28 @@ kbd {
   opacity: 1;
 }
 
+/* The Cancel button on an 'info' banner (e.g. the model-loading banner) — a real button
+   rather than the bare "×" close glyph above, since "Cancel" is an action with a
+   consequence (the caller's onCancel closes a session), not a plain dismiss. */
+.center-status-cancel {
+  flex-shrink: 0;
+  pointer-events: auto;
+  background: none;
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  color: inherit;
+  opacity: 0.75;
+  font-size: 0.8rem;
+  font-weight: 500;
+  padding: 0.25rem 0.6rem;
+  cursor: pointer;
+}
+
+.center-status-cancel:hover {
+  opacity: 1;
+  border-color: var(--text-muted, var(--border));
+}
+
 .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); }