fix: improve campaign stats, shared chart components, and i18n coverage

- Extract shared chart constants to constants/charts.ts
- Add TREND_STYLES to stats/constants.ts for reuse
- Fix useChartColors with MutationObserver for theme changes
- Fix DailyChart date parsing to prevent timezone shift
- Add truncate to StatCard value for overflow safety
- Fix CampaignCard copy feedback inside try block
- Add NaN validation and staleTime to AdminCampaignStats
- Replace hardcoded strings with i18n keys in Referral page
- Add missing i18n keys across all 4 locales (en, ru, zh, fa)
- Add sales stats route to AdminPanel and App router

src/pages/AdminPanel.tsx

@@ -414,6 +414,13 @@ export default function AdminPanel() {
           description: t('admin.panel.trafficUsageDesc'),
           permission: 'traffic:read',
         },
+        {
+          to: '/admin/sales-stats',
+          icon: <ChartBarIcon />,
+          title: t('admin.nav.salesStats'),
+          description: t('admin.panel.salesStatsDesc'),
+          permission: 'stats:read',
+        },
       ],
     },
     {