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

patch-proxy.js

@@ -0,0 +1,8 @@
+const fs = require('fs');
+let file = fs.readFileSync('src/routes/proxy.tsx', 'utf8');
+
+// The Proxy layout also uses heavy cards
+file = file.replace(/<Card>/g, '<Card className="bg-background border border-white/5 shadow-none">');
+
+fs.writeFileSync('src/routes/proxy.tsx', file);
+