feat(usage): refactor dashboard frontend with shadcn/ui components, skeleton states, and dark-theme charts

src/components/ui/skeleton.tsx

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+import { cn } from '@/lib/utils';
+
+function Skeleton({ className, ...props }: React.HTMLAttributes<HTMLDivElement>) {
+  return <div className={cn('bg-muted animate-pulse rounded-md', className)} {...props} />;
+}
+
+export { Skeleton };

src/components/usage/ChartCard.tsx

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+import React from 'react';
+import { Card, CardContent } from '@/components/ui/card';
+import { Skeleton } from '@/components/ui/skeleton';
+import { Button } from '@/components/ui/button';
+import { AlertTriangle, BarChart3, RotateCcw } from 'lucide-react';
+
+interface ChartStatesProps {
+  isLoading: boolean;
+  error: Error | null;
+  isEmpty: boolean;
+  onRetry: () => void;
+  title: string;
+  emptyTitle: string;
+  emptyDescription: string;
+  errorMessage: string;
+  retryLabel: string;
+  children: React.ReactNode;
+}
+
+export const ChartCard: React.FC<ChartStatesProps> = ({
+  isLoading,
+  error,
+  isEmpty,
+  onRetry,
+  title,
+  emptyTitle,
+  emptyDescription,
+  errorMessage,
+  retryLabel,
+  children,
+}) => {
+  return (
+    <Card className="border-border/60 bg-card/60 flex flex-col">
+      <CardContent className="flex flex-1 flex-col p-4">
+        <div className="mb-3 flex items-center justify-between">
+          <h3 className="text-foreground text-sm font-semibold tracking-tight">{title}</h3>
+        </div>
+        <div className="flex-1">
+          {isLoading ? (
+            <div className="flex h-[280px] flex-col gap-3">
+              <Skeleton className="h-full w-full" />
+            </div>
+          ) : error ? (
+            <div className="flex h-[280px] flex-col items-center justify-center gap-3">
+              <AlertTriangle className="text-destructive h-5 w-5" />
+              <p className="text-muted-foreground text-sm">{errorMessage}</p>
+              <Button variant="outline" size="sm" onClick={onRetry}>
+                <RotateCcw className="mr-1.5 h-3.5 w-3.5" />
+                {retryLabel}
+              </Button>
+            </div>
+          ) : isEmpty ? (
+            <div className="flex h-[280px] flex-col items-center justify-center gap-2">
+              <BarChart3 className="text-muted-foreground/50 h-6 w-6" />
+              <p className="text-foreground text-sm font-medium">{emptyTitle}</p>
+              <p className="text-muted-foreground max-w-[240px] text-center text-xs">
+                {emptyDescription}
+              </p>
+            </div>
+          ) : (
+            children
+          )}
+        </div>
+      </CardContent>
+    </Card>
+  );
+};

src/components/usage/CustomTooltip.tsx

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+import React from 'react';
+
+interface TooltipPayloadItem {
+  name?: string;
+  value?: number;
+  color?: string;
+}
+
+interface CustomTooltipProps {
+  active?: boolean;
+  payload?: TooltipPayloadItem[];
+  label?: string;
+}
+
+export const CustomTooltip: React.FC<CustomTooltipProps> = ({ active, payload, label }) => {
+  if (!active || !payload || payload.length === 0) {
+    return null;
+  }
+
+  return (
+    <div className="bg-popover border-border rounded-md border px-3 py-2 shadow-md">
+      <p className="text-foreground mb-1.5 text-xs font-medium">{label}</p>
+      <div className="flex flex-col gap-1">
+        {payload.map((entry, index) => (
+          <div key={index} className="flex items-center gap-2 text-xs">
+            <span className="h-2 w-2 rounded-full" style={{ backgroundColor: entry.color }} />
+            <span className="text-muted-foreground">{entry.name}:</span>
+            <span className="text-foreground font-mono font-medium">
+              {typeof entry.value === 'number' ? entry.value.toLocaleString() : entry.value}
+            </span>
+          </div>
+        ))}
+      </div>
+    </div>
+  );
+};

src/components/usage/StatCard.tsx

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+import React from 'react';
+import { Card, CardContent } from '@/components/ui/card';
+import { Skeleton } from '@/components/ui/skeleton';
+import { cn } from '@/lib/utils';
+import type { LucideIcon } from 'lucide-react';
+
+interface StatCardProps {
+  label: string;
+  value: number;
+  icon: LucideIcon;
+  isLoading?: boolean;
+  className?: string;
+  accent?: 'blue' | 'green' | 'amber' | 'slate';
+}
+
+const ACCENT_MAP = {
+  blue: 'text-blue-400',
+  green: 'text-emerald-400',
+  amber: 'text-amber-400',
+  slate: 'text-slate-400',
+};
+
+const BG_MAP = {
+  blue: 'bg-blue-500/5',
+  green: 'bg-emerald-500/5',
+  amber: 'bg-amber-500/5',
+  slate: 'bg-slate-500/5',
+};
+
+export const StatCard: React.FC<StatCardProps> = ({
+  label,
+  value,
+  icon: Icon,
+  isLoading,
+  className,
+  accent = 'slate',
+}) => {
+  return (
+    <Card
+      className={cn(
+        'border-border/60 bg-card/60 hover:bg-card backdrop-blur-none transition-colors',
+        className,
+      )}
+    >
+      <CardContent className="flex items-center justify-between p-4">
+        <div className="flex flex-col gap-1">
+          <span className="text-muted-foreground text-xs font-medium tracking-wide uppercase">
+            {label}
+          </span>
+          {isLoading ? (
+            <Skeleton className="h-7 w-24" />
+          ) : (
+            <span className="text-foreground font-mono text-2xl font-semibold tracking-tight">
+              {value.toLocaleString()}
+            </span>
+          )}
+        </div>
+        <div
+          className={cn(
+            'flex h-9 w-9 shrink-0 items-center justify-center rounded-md',
+            BG_MAP[accent],
+          )}
+        >
+          <Icon className={cn('h-4 w-4', ACCENT_MAP[accent])} />
+        </div>
+      </CardContent>
+    </Card>
+  );
+};