WS3 frontend: local model picker, availability, and preferences

Surfaces the new "local" provider in the model picker, gated purely on
server-reported configuration (never a per-user key):

- mikeApi.ts: ApiKeyStatus gains optional local?: {configured, models}
  (additive; ApiKeyProvider deliberately untouched - no BYO local key).
- UserProfileContext.tsx: UserProfile gains localModels: string[].
- ModelToggle.tsx: "Local (on-premises)" group merged in at runtime from
  server-reported models, listed last, hidden entirely when unconfigured;
  group header links to docs/local-models.md, each row hints "runs on your
  own hardware".
- modelAvailability.ts: ModelProvider gains "local"; availability is
  server-reported (localModels), never API-key-gated.
- useSelectedModel.ts: accepts localModels; a persisted local: selection
  is re-validated against the reported list and falls back to
  DEFAULT_MODEL_ID gracefully when no longer valid.
- ChatInput.tsx, TabularReviewView.tsx, TRChatPanel.tsx,
  account/models/page.tsx: localModels threaded from useUserProfile()
  through every isModelAvailable/ModelToggle call site (chat, tabular
  chat, tabular generation gating, title/tabular account preferences).

ApiKeyMissingModal needed no changes: it only fires when isModelAvailable
returns false, which a hidden (unconfigured) local model never triggers.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>