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/panels-ui.js

@@ -5567,9 +5567,16 @@ Object.assign(CodemanApp.prototype, {
    * close button, since a sticky error the user cannot dismiss would just sit there). The
    * DOM is rebuilt fresh each call rather than patched, since which children exist differs
    * by type; `setMessage` still only ever touches the text node afterwards.
+   *
+   * `opts.onCancel` — when given (any type, but in practice only 'info': an 'error' banner
+   * already has its own close button), renders a "Cancel" button that calls it on click.
+   * The callback owns everything that follows (dismissing the banner, stopping whatever
+   * loop this was showing progress for, closing a session it was for) — this helper only
+   * renders the button and wires the click, the same "caller decides what cancel means"
+   * split as `_confirmModelSwap`'s promise-resolving buttons.
    */
   _showCenterStatus(message, opts = {}) {
-    const { type = 'info' } = opts;
+    const { type = 'info', onCancel } = opts;
     let el = document.getElementById('customModelCenterStatus');
     if (!el) {
       el = document.createElement('div');
@@ -5604,6 +5611,15 @@ Object.assign(CodemanApp.prototype, {
         dismiss();
       };
       el.appendChild(closeBtn);
+    } else if (onCancel) {
+      const cancelBtn = document.createElement('button');
+      cancelBtn.className = 'center-status-cancel';
+      cancelBtn.textContent = 'Cancel';
+      cancelBtn.onclick = (e) => {
+        e.stopPropagation();
+        onCancel();
+      };
+      el.appendChild(cancelBtn);
     }
     el.hidden = false;
     requestAnimationFrame(() => el.classList.add('show'));