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/components/stats/constants.ts

@@ -0,0 +1,5 @@
+export const TREND_STYLES = {
+  up: { arrow: '\u2191', className: 'text-success-400' },
+  down: { arrow: '\u2193', className: 'text-error-400' },
+  stable: { arrow: '\u2192', className: 'text-dark-400' },
+} as const;