feat(api): add stats chart endpoint for admin access with time series data

feat(dayjs): integrate dayjs for date handling and formatting across the application
refactor(routes): update date handling in id-tags, transactions, users, and dashboard routes to use dayjs
style(globals): improve CSS variable definitions for better readability and consistency
deps: add dayjs as a dependency for date manipulation

apps/web/lib/api.ts

@@ -154,9 +154,21 @@ export type PaginatedTransactions = {
 
 // ── API functions ──────────────────────────────────────────────────────────
 
+export type ChartRange = "30d" | "7d" | "24h";
+
+export type ChartDataPoint = {
+  bucket: string;
+  energyKwh: number;
+  revenue: number;
+  transactions: number;
+  utilizationPct: number;
+};
+
 export const api = {
   stats: {
     get: () => apiFetch<Stats | UserStats>("/api/stats"),
+    chart: (range: ChartRange) =>
+      apiFetch<ChartDataPoint[]>(`/api/stats/chart?range=${range}`),
   },
   chargePoints: {
     list: () => apiFetch<ChargePoint[]>("/api/charge-points"),