Simplify auth, remove abstraction layers, add shadcn components

- Delete lib/api/errors.ts and lib/api/auth-guard.ts, inline
  NextResponse.json() error responses directly in route handlers
- Replace non-standard amber CSS variables with primary/primary-foreground
- Regenerate shadcn UI components with biome-ignore comments
- Add CLAUDE.md for repository guidance
- Update dependencies and pnpm lockfile

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

components.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://ui.shadcn.com/schema.json",
-  "style": "base-lyra",
+  "style": "base-mira",
   "rsc": true,
   "tsx": true,
   "tailwind": {