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>

app/api/highlights/all/route.ts

@@ -1,10 +1,10 @@
-import { NextResponse } from 'next/server'
+import { NextResponse, NextRequest } from 'next/server'
 import { prisma } from '@/lib/db'
 import { getAuth } from '@clerk/nextjs/server'
 
 export const runtime = 'nodejs'
 
-export async function GET(request: Request) {
+export async function GET(request: NextRequest) {
   try {
     const { userId } = await getAuth(request)
     if (!userId) {

app/api/highlights/bulk/route.ts

@@ -1,10 +1,10 @@
-import { NextResponse } from 'next/server'
+import { NextResponse, NextRequest } from 'next/server'
 import { prisma } from '@/lib/db'
 import { getAuth } from '@clerk/nextjs/server'
 
 export const runtime = 'nodejs'
 
-export async function POST(request: Request) {
+export async function POST(request: NextRequest) {
   try {
     const { userId } = await getAuth(request)
     if (!userId) {

app/api/highlights/route.ts

@@ -1,10 +1,10 @@
-import { NextResponse } from 'next/server'
+import { NextResponse, NextRequest } from 'next/server'
 import { prisma } from '@/lib/db'
 import { getAuth } from '@clerk/nextjs/server'
 
 export const runtime = 'nodejs'
 
-export async function POST(request: Request) {
+export async function POST(request: NextRequest) {
   try {
     const { userId } = await getAuth(request)
     if (!userId) {