fix: token tracking pipeline + premium usage dashboard redesign

Backend fixes:
- Fix totalTokens defaulting to 0 when API omits totalTokenCount (now computes prompt+completion)
- Fix streaming metadata extraction: unwrap response wrapper in Anthropic/Gemini handlers
- Fix Zod schemas: handle SQL SUM(null) with nullable().transform()
- Add is_estimated column migration to token_usage table

Frontend redesign:
- Rewrite Usage page with area charts (nivo/line), horizontal bar model ranking
- StatCard: add trend indicators (% vs previous period) and inline SVG sparklines
- Premium color palette, prompt/completion ratio bar, top models mini-list
- Live indicator with 15s auto-refresh

Design system migration:
- Standardize all components on Gemini Nexus tokens (rounded-xl, border-white/[0.06])
- Refine UI components, proxy tabs, cloud account list, provider groups

src/components/ui/skeleton.tsx

@@ -1,7 +1,12 @@
-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 };
+import { cn } from '@/lib/utils';
+
+function Skeleton({ className, ...props }: React.HTMLAttributes<HTMLDivElement>) {
+  return (
+    <div
+      className={cn('animate-pulse rounded-lg bg-white/[0.04]', className)}
+      {...props}
+    />
+  );
+}
+
+export { Skeleton };