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/admin/types.ts

@@ -66,6 +66,14 @@ export interface FeatureGates {
   crossStarredViewEnabled: boolean;
   crossAllViewEnabled: boolean;
   unifiedCrossAccountEnabled: boolean;
+  /**
+   * Master admin switch for the AI Assistant tab (docs/AI-ASSISTANT-CONCEPT.md).
+   * Defaults false, like pluginsEnabled — unlike every other gate, this one
+   * fronts a feature with no licensed provider class behind it yet (P1/P2 of
+   * that doc's phased rollout), so an admin opts in explicitly rather than
+   * every existing install suddenly showing a tab that does nothing.
+   */
+  aiAssistantEnabled: boolean;
 }
 
 export const DEFAULT_FEATURE_GATES: FeatureGates = {
@@ -92,6 +100,7 @@ export const DEFAULT_FEATURE_GATES: FeatureGates = {
   crossStarredViewEnabled: false,
   crossAllViewEnabled: false,
   unifiedCrossAccountEnabled: false,
+  aiAssistantEnabled: false,
 };
 
 export interface ThemePolicy {