fix: spread DarkTheme to include fonts in NavigationContainer theme

React Navigation v7 requires theme.fonts (medium, regular, bold, heavy).
Providing a partial theme without fonts caused BottomTabItem to crash
with "Cannot read property 'medium' of undefined".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mobile/App.js

@@ -1,6 +1,6 @@
 import React, { useEffect, useState, Component } from 'react';
 import { View, StatusBar, Text, ScrollView } from 'react-native';
-import { NavigationContainer } from '@react-navigation/native';
+import { NavigationContainer, DarkTheme } from '@react-navigation/native';
 import { createBottomTabNavigator } from '@react-navigation/bottom-tabs';
 import { createStackNavigator } from '@react-navigation/stack';
 import { SafeAreaProvider } from 'react-native-safe-area-context';
@@ -128,8 +128,9 @@ export default function App() {
         <SafeAreaProvider>
           <StatusBar barStyle="light-content" backgroundColor="#121212" />
           <NavigationContainer theme={{
-            dark: true,
+            ...DarkTheme,
             colors: {
+              ...DarkTheme.colors,
               primary:      '#1db954',
               background:   '#121212',
               card:         '#1e1e1e',