fix(mobile): correct asset logo path depth and add expo-router app/ routes for web support

- Fix fatal crash: require('../../../assets/logo.png') in LoginScreen and
  WelcomeScreen resolved 3 levels above mobile/src/screens/ — outside the
  mobile/ directory. Changed to ../../assets/logo.png which correctly
  resolves to the existing mobile/assets/logo.png.
- Add expo-router app/ directory (root cause of missing welcome screen and
  web support): app/_layout.tsx, (auth)/, (tabs)/ with all route files
- Add WelcomeScreen.tsx: branded intro screen with feature highlights
- Update LoginScreen/WelcomeScreen to use useRouter() (expo-router style)
- Add react-native-web ~0.20.0 and react-dom 19.2.4 for web channel
- Add expo-device ~7.0.3 (was imported but missing from package.json)
- Remove android.googleServicesFile from app.json (file is gitignored;
  README documents how to restore it for Android FCM builds)
- Add web.bundler: metro and web.output: single to app.json
- Fix aria-hidden to explicit boolean value in WelcomeScreen

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

mobile/app/(tabs)/_layout.tsx

@@ -0,0 +1,72 @@
+/**
+ * Tab navigator layout for authenticated users.
+ *
+ * Registers three tabs: Upload (default), Files, and Profile.
+ * Push notifications are initialised here so they activate as soon as the
+ * user enters the authenticated area.
+ */
+
+import { Tabs } from "expo-router";
+import { Ionicons } from "@expo/vector-icons";
+import React from "react";
+import { usePushNotifications } from "../../src/hooks/usePushNotifications";
+import { useAuth } from "../../src/context/AuthContext";
+
+export default function TabLayout() {
+  const { isAuthenticated } = useAuth();
+  usePushNotifications(isAuthenticated);
+
+  return (
+    <Tabs
+      screenOptions={{
+        tabBarActiveTintColor: "#1e40af",
+        tabBarInactiveTintColor: "#9ca3af",
+        tabBarStyle: {
+          borderTopColor: "#e5e7eb",
+          backgroundColor: "#ffffff",
+        },
+        headerStyle: {
+          backgroundColor: "#1e40af",
+        },
+        headerTintColor: "#ffffff",
+        headerTitleStyle: {
+          fontWeight: "700",
+        },
+      }}
+    >
+      <Tabs.Screen
+        name="index"
+        options={{
+          title: "Upload",
+          tabBarLabel: "Upload",
+          tabBarIcon: ({ color, size }) => (
+            <Ionicons name="cloud-upload-outline" size={size} color={color} />
+          ),
+          headerTitle: "DocuElevate",
+        }}
+      />
+      <Tabs.Screen
+        name="files"
+        options={{
+          title: "Files",
+          tabBarLabel: "Files",
+          tabBarIcon: ({ color, size }) => (
+            <Ionicons name="document-text-outline" size={size} color={color} />
+          ),
+          headerTitle: "My Documents",
+        }}
+      />
+      <Tabs.Screen
+        name="profile"
+        options={{
+          title: "Profile",
+          tabBarLabel: "Profile",
+          tabBarIcon: ({ color, size }) => (
+            <Ionicons name="person-circle-outline" size={size} color={color} />
+          ),
+          headerTitle: "Profile",
+        }}
+      />
+    </Tabs>
+  );
+}