feat: app mobile React Native / Expo (v0.31.0)

Package mobile/ (Expo SDK 56, expo-router) réutilisant l'API backend :
Login (mdp+TOTP), Chat (+ TTS expo-speech), Review (approuver/refuser).
JWT en expo-secure-store, thème dark HUD, EXPO_PUBLIC_API_BASE. typecheck
vert. STT mobile reporté (lib native), TTS OK. Versions gérées par Expo.

Palier de risque : reversible (client mobile, API commune).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

mobile/app/_layout.tsx

@@ -0,0 +1,18 @@
+import { Stack } from "expo-router";
+import { StatusBar } from "expo-status-bar";
+import { AuthProvider } from "@/auth";
+import { C } from "@/theme";
+
+export default function RootLayout() {
+  return (
+    <AuthProvider>
+      <StatusBar style="light" />
+      <Stack
+        screenOptions={{
+          headerShown: false,
+          contentStyle: { backgroundColor: C.bg },
+        }}
+      />
+    </AuthProvider>
+  );
+}