feat(custom-model): generate Run-menu entries from saved endpoint profiles

Follow-up to #393, picking up the work Ark0N invited in his merge comment:
"generate those entries from the saved profiles rather than a fixed
duplicate per harness, and put it in a follow-up PR so this one stays the
backend... The Run-menu picker is yours if you want it."

Adds the frontend surface the backend has been waiting on:

- Run menu: a "Custom Endpoints" section lists one entry per (harness that
  supports customModelInjection, saved endpoint) pair, e.g.
  "Claude Code (llama.cpp)". The harness list comes from
  window.__codemanCustomModelClis, injected at page render straight off the
  CLI registry's own capabilities (never a hardcoded id list in the
  frontend), so a CLI whose injection recipe lands later appears with no
  frontend change. Picking an entry runs that harness's own existing run*()
  function unmodified (case creation, env overrides, everything, forced to
  a single instance) and then applies the endpoint's default model to the
  session it creates via the existing POST /api/sessions/:id/custom-model
  route. Entries are hidden for a remote/docker active case, since that
  route already refuses both.
- Settings: App Settings -> Models gets a "Custom model endpoints" group
  wiring up the customModelEndpointsEnabled toggle (declared since #393,
  read by nothing until now) plus CRUD against the existing
  /api/model-endpoints routes: list, add/edit (inline form), delete,
  discover models.
- Backend: CustomModelHost gains an optional defaultModelId, the model the
  picker applies with no further choice per endpoint (one generated menu
  entry per CLI+endpoint pair, not per CLI+endpoint+model). The route
  refuses a value that isn't one of the endpoint's own discovered models,
  and a fresh discovery drops a default that no longer appears rather than
  carrying an invalid one forward.

Docs: docs/custom-model-endpoints.md describes the new picker and settings
panel; CLAUDE.md's Custom Model Endpoint Profiles entry drops the
"backend-only" status note and documents the picker's generation mechanism.

Tests: four new route tests cover defaultModelId validation, acceptance,
and the drop/keep behaviour across a re-discovery; a new render-index-html
test pins the __codemanCustomModelClis injection (present, agent CLIs
supporting the capability, antigravity and shell excluded) and its
solo-window skip. No browser test was added for the Run-menu picker itself
or the settings CRUD panel (this box has no tmux, so the live server used
by test:browser/test:mobile could not be exercised here) -- worth a
Playwright pass before merge, same as any other frontend PR.

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

src/web/schemas.ts

@@ -1918,6 +1918,12 @@ export const CustomModelHostSchema = z.object({
   authStyle: z.enum(['bearer', 'api-key']).optional(),
   models: z.array(z.string().max(200)).max(200).optional(),
   lastDiscoveredAt: z.string().max(64).optional(),
+  // The Run-menu picker's per-endpoint default; validated against `models` at the
+  // route layer (schema-level cross-field checks can't see the array narrowed the
+  // same way a `.refine()` closure could, and the route already re-reads the stored
+  // host to apply it, so the check belongs there once, not duplicated into a refine
+  // that would run on every unrelated field edit too).
+  defaultModelId: z.string().max(200).optional(),
 });
 
 /** POST /api/sessions/:id/custom-model — apply or clear a session's custom-model selection. */