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/_layout.tsx

@@ -0,0 +1,41 @@
+import { useEffect } from 'react'
+import { Slot, useRouter, useSegments } from 'expo-router'
+import { SafeAreaProvider } from 'react-native-safe-area-context'
+import { StatusBar } from 'expo-status-bar'
+import { View, ActivityIndicator } from 'react-native'
+import { useAuthStore } from '@/lib/store'
+
+export default function RootLayout() {
+  const { user, loading, restore } = useAuthStore()
+  const router = useRouter()
+  const segments = useSegments()
+
+  useEffect(() => {
+    restore()
+  }, [])
+
+  useEffect(() => {
+    if (loading) return
+    const inAuth = segments[0] === '(auth)'
+    if (!user && !inAuth) {
+      router.replace('/(auth)/login')
+    } else if (user && inAuth) {
+      router.replace('/(tabs)/home')
+    }
+  }, [user, loading, segments])
+
+  if (loading) {
+    return (
+      <View className="flex-1 items-center justify-center bg-paper">
+        <ActivityIndicator size="large" color="#A47148" />
+      </View>
+    )
+  }
+
+  return (
+    <SafeAreaProvider>
+      <StatusBar style="auto" />
+      <Slot />
+    </SafeAreaProvider>
+  )
+}