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/(main)/[locale]/settings/page.tsx

@@ -34,6 +34,7 @@ import {
   Bug,
   SwatchBook,
   Download,
+  Sparkles,
   X,
   type LucideIcon,
 } from 'lucide-react';
@@ -66,6 +67,7 @@ import { SidebarAppsSettings } from '@/components/settings/sidebar-apps-settings
 import { NotificationSettings } from '@/components/settings/notification-settings';
 import { ThemesSettings } from '@/components/settings/themes-settings';
 import { PluginsSettings } from '@/components/settings/plugins-settings';
+import { AiAssistantSettings } from '@/components/settings/ai-assistant-settings';
 import { PluginIframeSlot } from '@/components/plugins/plugin-iframe-slot';
 import { offersForSlot as pluginOffersForSlot, subscribe as pluginRegistrySubscribe, get as getActivePlugin } from '@/lib/plugin-sandbox/registry';
 import { ProtocolHandlerSettings } from '@/components/settings/protocol-handler-settings';
@@ -111,6 +113,7 @@ type Tab =
   | 'about_data'
   | 'themes'
   | 'plugins'
+  | 'ai_assistant'
   | 'debug';
 
 type TabGroup = 'general' | 'appearance' | 'mail' | 'privacy' | 'apps' | 'advanced';
@@ -153,6 +156,7 @@ const tabIcons: Record<Tab, LucideIcon> = {
   about_data: Info,
   themes: SwatchBook,
   plugins: Puzzle,
+  ai_assistant: Sparkles,
   debug: Bug,
 };
 
@@ -234,6 +238,7 @@ const tabSearchPaths: Record<Tab, string[]> = {
   about_data: ['settings.advanced'],
   themes: [],
   plugins: [],
+  ai_assistant: [],
   debug: ['settings.advanced'],
 };
 
@@ -264,6 +269,7 @@ const tabKeywords: Record<Tab, string> = {
   about_data: 'export import storage quota privacy backup',
   themes: 'custom theme css skin appearance',
   plugins: 'extensions addons',
+  ai_assistant: 'assistant ask model llm ollama chatbot',
   debug: 'logs developer console diagnostic',
 };
 
@@ -652,6 +658,7 @@ export default function SettingsPage() {
     // Advanced
     { id: 'about_data', label: t('tabs.about_data'), icon: tabIcons.about_data, group: 'advanced' },
     ...(isFeatureEnabled('pluginsEnabled') ? [{ id: 'plugins' as Tab, label: 'Plugins', icon: tabIcons.plugins, group: 'advanced' as TabGroup }] : []),
+    ...(isFeatureEnabled('aiAssistantEnabled') ? [{ id: 'ai_assistant' as Tab, label: 'AI Assistant', icon: tabIcons.ai_assistant, group: 'advanced' as TabGroup }] : []),
     ...(isFeatureEnabled('debugModeEnabled') ? [{ id: 'debug' as Tab, label: t('tabs.debug'), icon: tabIcons.debug, group: 'advanced' as TabGroup }] : []),
   ];
 
@@ -777,6 +784,7 @@ export default function SettingsPage() {
       {effectiveActiveTab === 'about_data' && <AboutDataSettings />}
       {effectiveActiveTab === 'themes' && <ThemesSettings />}
       {effectiveActiveTab === 'plugins' && <PluginsSettings />}
+      {effectiveActiveTab === 'ai_assistant' && <AiAssistantSettings />}
       {effectiveActiveTab === 'debug' && <DebugSettings />}
       {effectiveActiveTab.startsWith('plugin:') && (
         <PluginIframeSlot

app/(main)/admin/_tabs/policy.tsx

@@ -28,6 +28,7 @@ const FEATURE_GATE_LABELS: Partial<Record<keyof FeatureGates, { label: string; d
   crossStarredViewEnabled: { label: 'Unified Mailbox: Starred', description: 'Allow a "Starred" entry in the Unified Mailbox section that lists flagged/starred mail across the account and its shared folders (or every account when the cross-account sub-option is on). Honors the user\'s folder selection. Requires the matching per-user toggle in Settings → Appearance.' },
   crossAllViewEnabled: { label: 'Unified Mailbox: All Mail', description: 'Allow an "All mail" entry in the Unified Mailbox section that lists all mail across the account and its shared folders (or every account when the cross-account sub-option is on). Honors the user\'s folder selection. Requires the matching per-user toggle in Settings → Appearance.' },
   unifiedCrossAccountEnabled: { label: 'Unified Mailbox: Cross-account', description: 'Allow users to expand the Unified Mailbox beyond the active account boundary so its lists merge across every logged-in account. When off, the Unified Mailbox stays within the active account and its shared folders.' },
+  aiAssistantEnabled: { label: 'AI Assistant (preview)', description: 'Show the AI Assistant settings tab. No provider is wired up yet (see docs/AI-ASSISTANT-CONCEPT.md) — enabling this only proves the settings pane and policy fetch, it does not grant access to any model.' },
 };
 
 const RESTRICTABLE_SETTINGS = [

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 });
+  }
+}