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

docs/wiki/Custom-Model-Endpoints.md

@@ -80,14 +80,15 @@ just applying a selection never reached llama-swap's own logs at all until somet
 it to load. Picking an entry now also sends the smallest real request that will trigger
 that load, in the background, the moment the target model isn't already loaded and ready.
 
-**The centred loading banner shows a live countdown, and a real timeout is an error, not a
-shrug.** When it knows the model's discovered file size (its GB figure, when llama-swap
-states one), it shows both a rough expected-time estimate and a live countdown against it —
-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, the banner turns into a sticky
-error telling you to check the llama-swap server's own logs, and **the session that load was
-for is closed automatically** — a console left open and pointed at a model that never
-finished loading would just be confusing to leave sitting there.
+**The centred loading banner has no countdown and no automatic timeout — it waits as long as
+it takes, and tells you so.** When it knows the model's discovered file size (its GB figure,
+when llama-swap states one) it's shown too, e.g. "Loading qwen3.8-27b (16.4 GB) on
+llama-swap — this can take a while depending on your hardware and the model size." An
+earlier version tried to estimate and enforce a time limit, but real load time depends on
+hardware this feature has no way to know, so a fixed number was always a guess — worse, one
+that could kill a genuinely slow load partway through. If it really is taking too long, a
+**Cancel** button right on the banner ends the wait and **closes the session that load was
+for**, on your own call rather than a guessed deadline.
 
 **The banner also shows a real, live second line of what llama.cpp itself is doing** — not
 a made-up progress phase, the actual next line the `llama-server` process printed, e.g.