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.

app/api/ai/policy/route.ts

@@ -0,0 +1,33 @@
+import { NextResponse } from 'next/server';
+import { configManager } from '@/lib/admin/config-manager';
+import { logger } from '@/lib/logger';
+import { DEFAULT_AI_ENTITLEMENT, type AiPolicy } from '@/lib/ai/types';
+
+/**
+ * GET /api/ai/policy - AI Assistant policy (NOT admin-protected - users read this)
+ *
+ * P0 stub (docs/AI-ASSISTANT-CONCEPT.md §12): proves the client<->server
+ * policy-fetch plumbing end-to-end with no provider ever called. `enabled`
+ * mirrors the admin FeatureGates toggle; entitlement is hardcoded unlicensed
+ * until P2 wires a real seats/billing backend (§9) - there is no provider
+ * class to grant yet regardless of what an entitlement record might say.
+ */
+export async function GET() {
+  try {
+    await configManager.ensureLoaded();
+    const policy = configManager.getPolicy();
+
+    const aiPolicy: AiPolicy = {
+      enabled: policy.features.aiAssistantEnabled,
+      entitlement: { ...DEFAULT_AI_ENTITLEMENT },
+      publicConsentVersion: null,
+    };
+
+    return NextResponse.json(aiPolicy, {
+      headers: { 'Cache-Control': 'no-store' },
+    });
+  } catch (error) {
+    logger.error('AI policy read error', { error: error instanceof Error ? error.message : 'Unknown error' });
+    return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
+  }
+}