Remove duplicate back buttons from all Dashboard screens

mobile/src/screens/SettingsScreen.tsx

@@ -348,9 +348,7 @@ const SettingsScreen: React.FC = () => {
 
       {/* Header */}
       <View style={[styles.header, { backgroundColor: colors.surface, borderBottomColor: colors.border }]}>
-        <TouchableOpacity onPress={() => navigation.goBack()} style={styles.backButton}>
-          <Text style={styles.backButtonText}>←</Text>
-        </TouchableOpacity>
+        <View style={{ width: 40 }} />
         <Text style={[styles.headerTitle, { color: colors.text }]}>Settings</Text>
         <View style={{ width: 40 }} />
       </View>