fix(custom-model): address PR #430 pre-merge review (Ark0N)

Four blockers from the 2026-09-18 review:

- PUT /api/model-endpoints/:id now merges modelContextLengths/
  modelSizesGB back in from the stored record instead of trusting the
  editor's body, so renaming an endpoint or changing its default model
  no longer silently drops the context-window floor check and
  CLAUDE_CODE_MAX_CONTEXT_TOKENS injection.
- custom-model:swapped-out is now session-scoped (added to
  SESSION_PREFIXES) instead of broadcasting to every connected client.
- The quick-start custom-model path now hands setCustomModel() only
  the endpoint's own injected env vars, not the full merged set,
  matching the restart-in-place path — the full set put
  CLAUDE_CODE_EFFORT_LEVEL back after the Session constructor had
  already stripped it.
- The quick-start launchModel override for pi/grok/omp is now applied
  generically via the registry's legacyConfigField, mirroring
  Session._withCustomModelLaunchModel, instead of three hardcoded
  mode === '<id>' branches a future CLI's injection recipe would miss.

Also scopes the sticky-toast default (item 5): reverted the blanket
"all error toasts are sticky" default, which had no container cap or
eviction, back to a flat 3s; the one message that needs a moment to
read (a failed custom-model apply) now passes an explicit
duration: 0 at its own call site.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ea59JhUmHBm1gRCsiYF33R

src/web/public/session-ui.js

@@ -962,7 +962,9 @@ Object.assign(CodemanApp.prototype, {
     if (!ok || !data || data.success === false) {
       switchingToast?.dismiss();
       const detail = data?.error ? `: ${data.error}` : res ? ` (HTTP ${res.status})` : ' (request failed)';
-      this.showToast(`Session started on the native backend — could not apply the custom endpoint${detail}`, 'error');
+      this.showToast(`Session started on the native backend — could not apply the custom endpoint${detail}`, 'error', {
+        duration: 0,
+      });
       return;
     }