fix(custom-model,toast): surface the real apply error, and make error toasts sticky with a close button

Two related fixes, both needed to actually diagnose 'Session started on
the native backend — could not apply the custom endpoint' reports from
live testing:

1. runCustomModelEntry()'s apply call went through _apiJson(), which
   unwraps a success body but SWALLOWS a failure response entirely and
   returns null — discarding the one thing (error, errorCode) that would
   tell 'endpoint unreachable' apart from 'not a discovered model',
   'remote/Docker session', or a dozen other real causes the apply route
   already reports distinctly. Switched to _api() so the actual response
   body is read on failure too, and the toast now includes the real
   message.
2. showToast() defaulted every toast, error or not, to a 3s auto-dismiss
   with no way to read it again — exactly what made the above generic
   message impossible to act on even before the fix above. Error toasts
   now default to sticky (duration: 0, no auto-dismiss) unless a caller
   opts into a duration, and every toast — sticky or not — gets an
   explicit close (x) button, since a sticky toast with no way to
   dismiss it would just accumulate across repeated failures.

Tests: custom-model-run-menu-ui.test.ts's two apply tests updated for the
_api() switch (their mocks previously stubbed _apiJson, which the apply
call no longer goes through), plus a new test pinning that the real
server error string reaches the toast on a failure.

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

src/web/public/styles.css

@@ -8463,6 +8463,9 @@ kbd {
 }
 
 .toast {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
   background: var(--bg-card);
   border: 1px solid var(--border);
   border-radius: 6px;
@@ -8474,6 +8477,7 @@ kbd {
   opacity: 0;
   transition: all 0.2s ease;
   pointer-events: auto;
+  max-width: 420px;
 }
 
 .toast.show {
@@ -8481,6 +8485,32 @@ kbd {
   opacity: 1;
 }
 
+.toast-message {
+  flex: 1;
+  /* Errors are sticky by default (showToast) precisely so a longer, specific
+     message survives to be read — let it wrap instead of clipping. */
+  white-space: pre-wrap;
+  word-break: break-word;
+}
+
+/* Every toast gets one, sticky or not: a sticky toast with no way to close it
+   would just accumulate on screen across repeated failures. */
+.toast-close {
+  flex-shrink: 0;
+  background: none;
+  border: none;
+  color: inherit;
+  opacity: 0.6;
+  font-size: 1.1rem;
+  line-height: 1;
+  padding: 0 0.15rem;
+  cursor: pointer;
+}
+
+.toast-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); }