feat: VAT toggle + offline mode

- Rename Revenue to GrossRevenue, add NetRevenue (excl. VAT)
- Add VAT toggle (Incl/Excl) on Dashboard and Comparison pages
- Add offline mode with localStorage caching (24h validity)
- Add refresh button and offline indicator in nav
- Remove Google Sheets fallback (archived to dataService.legacy.js)
- Add AR/EN translations for new UI elements

src/locales/ar.json

@@ -3,7 +3,9 @@
     "name": "هاي هلا داتا",
     "loading": "جارٍ تحميل البيانات...",
     "error": "تعذر تحميل البيانات",
-    "retry": "إعادة المحاولة"
+    "retry": "إعادة المحاولة",
+    "offline": "غير متصل",
+    "refresh": "تحديث البيانات"
   },
   "nav": {
     "dashboard": "لوحة التحكم",
@@ -13,11 +15,14 @@
     "labels": "التسميات",
     "labelsOn": "التسميات مفعّلة",
     "labelsOff": "التسميات معطّلة",
-    "labelsTooltip": "عرض القيم على الرسوم البيانية"
+    "labelsTooltip": "عرض القيم على الرسوم البيانية",
+    "vat": "الضريبة"
   },
   "toggle": {
     "on": "تشغيل",
-    "off": "إيقاف"
+    "off": "إيقاف",
+    "incl": "شامل",
+    "excl": "بدون"
   },
   "dataSources": {
     "museums": "المتاحف",

src/locales/en.json

@@ -3,7 +3,9 @@
     "name": "HiHala Data",
     "loading": "Loading data...",
     "error": "Unable to load data",
-    "retry": "Retry"
+    "retry": "Retry",
+    "offline": "Offline",
+    "refresh": "Refresh data"
   },
   "nav": {
     "dashboard": "Dashboard",
@@ -13,11 +15,14 @@
     "labels": "Labels",
     "labelsOn": "Labels On",
     "labelsOff": "Labels Off",
-    "labelsTooltip": "Show values on charts"
+    "labelsTooltip": "Show values on charts",
+    "vat": "VAT"
   },
   "toggle": {
     "on": "On",
-    "off": "Off"
+    "off": "Off",
+    "incl": "Incl",
+    "excl": "Excl"
   },
   "dataSources": {
     "museums": "Museums",