feat(ai): P0 client scaffolding — capability flags, settings pane, policy fetch

Per docs/AI-ASSISTANT-CONCEPT.md §12, P0 is deliberately generation-free:
prove platform gating and the policy round trip before any model exists
behind it. No provider is called anywhere in this change.

- lib/platform-capabilities.ts: supportsLocalLlm/localLlmNeedsCorsSetup,
  mirroring the same-named module in vncmail-native so the capability
  contract (§3, §11) reads identically on both clients. Web+Electron only
  here — mobile is a separate codebase.
- lib/ai/types.ts: AiPolicy/AiEntitlement schema, locked in now per decision
  #4 (entitlement from day one — cheap now, a live-tenant migration later).
- app/api/ai/policy/route.ts: GET, unauthenticated (users read this, like
  /api/admin/policy). Composes the real FeatureGates.aiAssistantEnabled
  toggle with a hardcoded unlicensed entitlement — there's no seats/billing
  backend yet (P2), so nothing here can honestly claim otherwise.
- components/settings/ai-assistant-settings.tsx: fetches that policy, shows
  a real (not fake) locked/unlicensed state. No model config UI yet — there
  is nothing real to configure until P1/P2/P5 land.
- New admin FeatureGates.aiAssistantEnabled (default false, like
  pluginsEnabled): the tab is entirely hidden until an admin opts in, so no
  existing install suddenly sees a tab that does nothing.

Verified: typecheck clean, lint clean, translations test passes (48/48),
full production build succeeds with /api/ai/policy compiled in.

lib/platform-capabilities.ts

@@ -0,0 +1,29 @@
+// Single source of truth for "this feature only exists on some platforms" —
+// mirrors the same-named module in vncmail-native (mobile), so the AI
+// Assistant capability contract (docs/AI-ASSISTANT-CONCEPT.md §3, §11)
+// reads identically across both clients.
+
+import { isElectronShell } from '@/lib/electron-bridge';
+
+/**
+ * The `local` provider class (a loopback Ollama-compatible runtime) needs a
+ * host process reachable on 127.0.0.1. Electron's main process can fetch
+ * loopback directly, no CORS constraint. A browser page can too — localhost
+ * is a trustworthy origin so mixed-content doesn't block it — but only if
+ * the runtime's own CORS allowlist permits this origin (see
+ * localLlmNeedsCorsSetup below). Mobile has neither the runtime nor the RAM
+ * and is a separate codebase (vncmail-native), not reachable from here.
+ */
+export function supportsLocalLlm(): boolean {
+  return true; // web or Electron — this codebase is never mobile
+}
+
+/**
+ * True only for the plain-browser case: Electron reaches loopback from its
+ * main process with no CORS involved at all, so this is specifically the
+ * "advise the user to set OLLAMA_ORIGINS" case (docs/AI-ASSISTANT-CONCEPT.md
+ * §3's note under the platform matrix), not a general capability check.
+ */
+export function localLlmNeedsCorsSetup(): boolean {
+  return !isElectronShell();
+}