feat(custom-model): ask which model on launch when an endpoint has more than one, and re-discover models every 5 minutes

Two enhancements requested after live-validating PR #430 against a real
llama.cpp server:

1. Model picker dialog. Picking a Run-menu Custom Endpoints entry used to
   apply the endpoint's defaultModelId (or the first discovered model)
   silently. Now, via the new selectCustomModelEntry() (session-ui.js):
   - exactly one discovered model launches straight away, same as before
   - two or more open a new #customModelPickModal listing every discovered
     model; defaultModelId (if set) is marked but never auto-chosen, since
     the point of asking is letting ONE launch deliberately differ from
     the saved default, not just confirming it
   The endpoint is re-fetched at click time rather than trusting anything
   cached from the dropdown's own render, since the model list can have
   changed (the sweep below, or a settings-panel edit) since it opened.
   runCustomModelEntry() itself — the actual launch, routed through run()
   for the in-flight lock, snapshot-guarded against applying to the wrong
   session — is unchanged; it now just always receives an explicit model
   id from one of these two paths instead of computing one itself.

2. Periodic re-discovery. Every saved endpoint's models now refresh
   automatically every 5 minutes in the background
   (CUSTOM_MODEL_REDISCOVER_INTERVAL_MS, server.ts, registered the same way
   as the Codex plan-usage poll it sits beside — this.cleanup.setInterval,
   off under testMode), so a model the server starts or stops serving shows
   up without another manual "Discover" click. The manual POST
   .../discover-models route and the new refreshAllCustomModelHosts()
   sweep (custom-model-routes.ts) now share one pure merge step
   (applyDiscoveredModels: stamps lastDiscoveredAt, drops a defaultModelId
   that no longer appears) rather than two copies that could drift. The
   sweep is best-effort per host — one endpoint being unreachable on a
   cycle never blocks the others — and re-reads the store before each
   host's write, keyed by id, so a concurrent edit or delete from the
   settings panel always wins over a sweep that started before it.

Tests: test/custom-model-endpoint-rediscovery.test.ts is a new, dedicated
file for the sweep (kept separate from custom-model-routes.test.ts because
that file's data dir is shared across every test in it — one temp HOME per
FILE, not per test — which would make a sweep-touches-every-host assertion
meaningless there). test/custom-model-run-menu-ui.test.ts gained a new
describe block driving the real picker modal through JSDOM: single-model
bypass, multi-model dialog with the default marked-not-chosen, picking a
row closes the modal and launches with that exact model, the endpoint
re-fetch, and the two "vanished by click time" toast paths.

Docs: docs/custom-model-endpoints.md, docs/wiki/Custom-Model-Endpoints.md,
docs/api-reference.md and CLAUDE.md's dense feature paragraph all updated
— the last of these also caught up two sentences that had gone stale after
the draft-review fixes landed (the picker routes through run() now, not a
raw run*() call).

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

docs/wiki/Custom-Model-Endpoints.md

@@ -16,20 +16,32 @@ Still in App Settings → Models → Custom model endpoints:
    say). An API key is optional; most local servers don't check one.
 2. **Discover** — fetches the endpoint's own model list over `GET /v1/models` and stores it.
 3. Pick a **default model** from what was discovered. This is the model the Run-menu entry
-   below applies with no further choice, so set it once you know which one you want.
+   applies directly when only one model is discovered; with two or more, it's just the one
+   pre-marked in the picker dialog described below, not a silent default.
 
 Endpoint management is admin-only in multi-user mode, the same as remote hosts and Docker
 hosts — these are machine-level infra, not a per-user setting.
 
+**Model lists refresh themselves.** Every saved endpoint is re-discovered automatically every
+5 minutes in the background, so a model the server starts serving later — or stops serving —
+shows up without another manual click of **Discover**. One endpoint being unreachable on a
+given cycle (powered off, wrong network) never blocks the others from refreshing.
+
 ## Running a session against one
 
-With the setting on and at least one endpoint carrying a usable default model, the **Run**
+With the setting on and at least one endpoint carrying a discovered model, the **Run**
 dropdown grows a **Custom Endpoints** section: one entry per harness that can redirect to a
 custom endpoint, per saved endpoint, e.g. "Claude Code (llama.cpp)". Picking one starts a
-session on that harness exactly the way its own entry would, then points it at the
-endpoint's default model. It is a one-off "try this endpoint" action, not a sticky mode — the
-plain **Run** button still means "this harness, native cloud" afterward, and a fresh session
-never inherits whatever the last one was pointed at.
+session on that harness exactly the way its own entry would. It is a one-off "try this
+endpoint" action, not a sticky mode — the plain **Run** button still means "this harness,
+native cloud" afterward, and a fresh session never inherits whatever the last one was
+pointed at.
+
+**Which model it uses depends on how many the endpoint has discovered.** With exactly one,
+the session launches straight away on that model — nothing to choose. With two or more, a
+small dialog asks which one to use for this launch before starting the session; the
+endpoint's default model, if set, is marked but not auto-picked, so a launch can deliberately
+use a different one without changing the saved default.
 
 Applying a selection **restarts the harness's process in place** — same tab, same
 conversation where the harness supports resuming one, fresh environment. That restart is