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/routes/index.tsx

@@ -35,9 +35,9 @@ function HomePageErrorBoundary({ error, reset }: { error: unknown; reset: () =>
 
   return (
     <div className="container mx-auto max-w-4xl p-6">
-      <div className="rounded-lg border border-dashed p-8 text-center">
-        <div className="text-lg font-semibold">{t('error.generic')}</div>
-        <div className="text-muted-foreground mt-2 text-sm">{t('action.retry')}</div>
+      <div className="rounded-xl border border-dashed border-white/[0.1] p-8 text-center">
+        <div className="text-base font-semibold">{t('error.generic')}</div>
+        <div className="text-muted-foreground mt-2 text-[13px]">{t('action.retry')}</div>
         <Button className="mt-4" variant="outline" onClick={() => reset()}>
           <RefreshCw className="mr-2 h-4 w-4" />
           {t('action.retry')}