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/constants/charts.ts

@@ -0,0 +1,22 @@
+export const CHART_COMMON = {
+  KOPEKS_DIVISOR: 100,
+  GRADIENT: {
+    START_OFFSET: '5%',
+    END_OFFSET: '95%',
+    START_OPACITY: 0.3,
+    END_OPACITY: 0,
+  },
+  AXIS: {
+    TICK_FONT_SIZE: 11,
+  },
+  TOOLTIP: {
+    BORDER_RADIUS: '8px',
+    FONT_SIZE: '12px',
+  },
+  STROKE_WIDTH: 2,
+  GRID_DASH: '3 3',
+  STALE_TIME: 60_000,
+  CHART: {
+    MARGIN: { top: 5, right: 10, left: 0, bottom: 5 },
+  },
+} as const;