feat: add navigation with tab bar and screen layouts

Expo Router file-based navigation with glass tab bar,
3 tabs (Créations, Explorer, Profil), Phosphor icons,
modal picker, and fade transitions.

app/_layout.tsx

@@ -0,0 +1,48 @@
+import { useEffect } from "react";
+import { StatusBar } from "expo-status-bar";
+import { Stack } from "expo-router";
+import { GestureHandlerRootView } from "react-native-gesture-handler";
+import { StyleSheet } from "react-native";
+import { useWallpaperStore } from "@/stores/wallpaper.store";
+import { useSettingsStore } from "@/stores/settings.store";
+import { colors } from "@/theme";
+
+export default function RootLayout() {
+  const loadWallpapers = useWallpaperStore((s) => s.loadSavedWallpapers);
+  const loadSettings = useSettingsStore((s) => s.loadSettings);
+
+  useEffect(() => {
+    loadWallpapers();
+    loadSettings();
+  }, []);
+
+  return (
+    <GestureHandlerRootView style={styles.root}>
+      <StatusBar style="light" />
+      <Stack
+        screenOptions={{
+          headerShown: false,
+          contentStyle: { backgroundColor: colors.background },
+          animation: "fade",
+        }}
+      >
+        <Stack.Screen name="(tabs)" />
+        <Stack.Screen
+          name="picker"
+          options={{ animation: "slide_from_bottom", presentation: "modal" }}
+        />
+        <Stack.Screen
+          name="editor/[id]"
+          options={{ animation: "fade" }}
+        />
+      </Stack>
+    </GestureHandlerRootView>
+  );
+}
+
+const styles = StyleSheet.create({
+  root: {
+    flex: 1,
+    backgroundColor: colors.background,
+  },
+});