Add back button to dashboard header

The dashboard is a hidden tab screen with the tab bar hidden, so on iOS
there was no way to return home (no hardware back, no swipe gesture on
tabs). Match the construction-site header pattern.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/(protected)/dashboard/index.tsx

@@ -2,7 +2,8 @@ import React, { useState, useEffect, useMemo, useRef } from 'react';
 import { View, Text, ScrollView, TouchableOpacity, ActivityIndicator, Dimensions } from 'react-native';
 import { SafeAreaView } from 'react-native-safe-area-context';
 import { StatusBar } from 'expo-status-bar';
-import { ChevronDown, ChevronUp, LayoutDashboard } from 'lucide-react-native';
+import { useRouter } from 'expo-router';
+import { ChevronDown, ChevronLeft, ChevronUp, LayoutDashboard } from 'lucide-react-native';
 import api from '@/utils/api';
 import EchartWrapper from '@/components/EchartWrapper';
 import {
@@ -22,6 +23,7 @@ const TOOLTIP_BASE = {
 const screenWidth = Dimensions.get("window").width;
 
 export default function DashboardScreen() {
+    const router = useRouter();
 
     const [chartData, setChartData] = useState<any>({
         costiRicavi: null,
@@ -206,7 +208,13 @@ export default function DashboardScreen() {
             
             <SafeAreaView edges={['top']} className="pt-5">
                 <View className="pb-6 px-6 z-10 flex-row justify-between items-center">
-                    <View>
+                    <TouchableOpacity
+                        onPress={() => router.navigate('/')}
+                        className="p-2 -ml-2 rounded-full active:bg-white/20 w-12 items-center justify-center"
+                    >
+                        <ChevronLeft size={28} color="white" pointerEvents="none" />
+                    </TouchableOpacity>
+                    <View className="flex-1 px-2">
                         <Text className="text-white text-md font-semibold uppercase tracking-wider mb-1">Grafici e Statistiche</Text>
                         <Text className="text-yellow-400 text-3xl font-bold leading-tight">Dashboard</Text>
                     </View>