feat: App mobile Expo + API mobile dédiée

memento-mobile/ (Expo + React Native + expo-router):
- Auth: login email/password → Bearer token (expo-secure-store)
- Layout: guard auth → redirect /(auth)/login ou /(tabs)/home
- Tabs: Accueil, Carnets, Recherche, Profil
- Screens: login, home (recent notes + quick actions), notebooks list,
  note viewer (WebView HTML), search (texte), notebook detail, profile
- Design: tokens brand-accent (#A47148), ink, concrete, paper, border
- lib/config.ts: API_URL dev/prod configurable
- lib/api.ts: apiFetch avec Bearer token automatique
- lib/store.ts: Zustand auth store (login/logout/restore)

memento-note/ (API mobile dédiée):
- lib/mobile-auth.ts: createMobileToken / verifyMobileToken (HMAC-SHA256, 90j)
- POST /api/mobile/auth/login: email+password → token + user
- GET /api/mobile/auth/me: valider token, retourner profil
- GET /api/mobile/notebooks: liste carnets avec nb notes
- GET /api/mobile/notes: notes récentes (filtre par carnet optionnel)
- GET /api/mobile/notes/[id]: contenu complet d'une note
- GET /api/mobile/search: recherche fulltext titre+contenu

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

memento-mobile/app/(tabs)/_layout.tsx

@@ -0,0 +1,55 @@
+import { Tabs } from 'expo-router'
+import { BookOpen, Search, Home, User } from 'lucide-react-native'
+
+export default function TabsLayout() {
+  return (
+    <Tabs
+      screenOptions={{
+        headerShown: false,
+        tabBarActiveTintColor: '#A47148',
+        tabBarInactiveTintColor: '#8A8A82',
+        tabBarStyle: {
+          backgroundColor: '#FAFAF8',
+          borderTopColor: '#E8E6E0',
+          borderTopWidth: 1,
+          paddingBottom: 4,
+          height: 60,
+        },
+        tabBarLabelStyle: {
+          fontSize: 10,
+          fontWeight: '600',
+          marginTop: -2,
+        },
+      }}
+    >
+      <Tabs.Screen
+        name="home"
+        options={{
+          title: 'Accueil',
+          tabBarIcon: ({ color, size }) => <Home size={size} color={color} />,
+        }}
+      />
+      <Tabs.Screen
+        name="notebooks"
+        options={{
+          title: 'Carnets',
+          tabBarIcon: ({ color, size }) => <BookOpen size={size} color={color} />,
+        }}
+      />
+      <Tabs.Screen
+        name="search"
+        options={{
+          title: 'Recherche',
+          tabBarIcon: ({ color, size }) => <Search size={size} color={color} />,
+        }}
+      />
+      <Tabs.Screen
+        name="profile"
+        options={{
+          title: 'Profil',
+          tabBarIcon: ({ color, size }) => <User size={size} color={color} />,
+        }}
+      />
+    </Tabs>
+  )
+}