Adds GET /admin (single-page dashboard), GET /admin/api/overview, and
GET /admin/api/usage per spec §4: providers/redis/cache/groups summary
and per-user/group current-window usage+limits. Gated by Config.Admin
(default off, not registered when disabled) and UserConfig.Admin.
Gate order: disabled -> 404 fall-through; unauthenticated -> 401;
authenticated non-admin -> 403; admin -> serve, counted against the
admin's own request limits like any other route.
Adds small, read-only, mutex-guarded hooks: modelRegistry.snapshot(),
limiter.currentUsage()/redisStatus(), authStore.snapshot(). No secret
field (API key, digest, redis password) is ever read into any admin
response type; verified by a canary-literal redaction test.
HTML/CSS/JS ship as a single Go raw-string const (Yaegi forbids
go:embed); confirmed interpretable via make yaegi-check.
