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/drag-window-region.tsx

@@ -35,7 +35,7 @@ export default function DragWindowRegion({ title }: DragWindowRegionProps) {
     <div className="flex w-screen items-stretch justify-between">
       <div className="draglayer w-full">
         {title && !isMacOS && (
-          <div className="flex flex-1 p-2 text-xs whitespace-nowrap text-gray-400 select-none">
+          <div className="flex flex-1 p-2 text-xs whitespace-nowrap text-muted-foreground select-none">
             {title}
           </div>
         )}