build: complete Phase 2.1 implementation and verify build

- Verified all exports in highlight-manager.ts are correct
- Installed @clerk/nextjs dependency for API routes
- Fixed TypeScript errors in API routes (NextRequest type)
- Fixed MUI Grid component usage in highlights-tab.tsx (replaced with Box flexbox)
- Fixed HighlightColor type assertion in reading-view.tsx
- Build completed successfully with no TypeScript errors

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

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

package.json

@@ -24,6 +24,7 @@
   "license": "ISC",
   "description": "",
   "dependencies": {
+    "@clerk/nextjs": "^6.35.0",
     "@emotion/react": "^11.14.0",
     "@emotion/styled": "^11.14.1",
     "@fontsource/roboto": "^5.2.8",