feat: implement usage tracking for AI requests

Adds local token usage tracking for all AI providers. Usage data is
captured during stream processing and stored in a local database.
Includes a new Usage tab in the Providers dashboard to visualize
historical token consumption.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/shared/components/index.js

@@ -15,6 +15,7 @@ export { default as Header } from "./Header";
 export { default as Footer } from "./Footer";
 export { default as OAuthModal } from "./OAuthModal";
 export { default as ModelSelectModal } from "./ModelSelectModal";
+export { default as UsageStats } from "./UsageStats";
 
 // Layouts
 export * from "./layouts";