Setup Auth Provider

// src/lib/auth.ts
import { supabase } from './supabase'

export async function signInWithEmail(email: string, password: string) {
  const { data, error } = await supabase.auth.signInWithPassword({
    email,
    password,
  })
  if (error) throw error
  return data
}

export async function signUp(email: string, password: string, fullName: string) {
  const { data, error } = await supabase.auth.signUp({
    email,
    password,
    options: {
      data: { full_name: fullName }
    }
  })
  if (error) throw error
  return data
}

export async function signInWithGoogle() {
  const { data, error } = await supabase.auth.signInWithOAuth({
    provider: 'google',
    options: {
      redirectTo: `${window.location.origin}/auth/callback`
    }
  })
  if (error) throw error
  return data
}

export async function signInWithLine() {
  const { data, error } = await supabase.auth.signInWithOAuth({
    provider: 'line' as any, // LINE needs custom setup
    options: {
      redirectTo: `${window.location.origin}/auth/callback`
    }
  })
  if (error) throw error
  return data
}

export async function signOut() {
  const { error } = await supabase.auth.signOut()
  if (error) throw error
}

export async function getCurrentUser() {
  const { data: { user } } = await supabase.auth.getUser()
  return user
}

Auth Context

// src/providers/auth-provider.tsx
'use client'

import { createContext, useContext, useEffect, useState } from 'react'
import { User, Session } from '@supabase/supabase-js'
import { supabase } from '@/lib/supabase'

interface AuthContextType {
  user: User | null
  session: Session | null
  isLoading: boolean
}

const AuthContext = createContext<AuthContextType>({
  user: null,
  session: null,
  isLoading: true,
})

export function AuthProvider({ children }: { children: React.ReactNode }) {
  const [user, setUser] = useState<User | null>(null)
  const [session, setSession] = useState<Session | null>(null)
  const [isLoading, setIsLoading] = useState(true)

  useEffect(() => {
    // Get initial session
    supabase.auth.getSession().then(({ data: { session } }) => {
      setSession(session)
      setUser(session?.user ?? null)
      setIsLoading(false)
    })

    // Listen for changes
    const { data: { subscription } } = supabase.auth.onAuthStateChange(
      (_event, session) => {
        setSession(session)
        setUser(session?.user ?? null)
      }
    )

    return () => subscription.unsubscribe()
  }, [])

  return (
    <AuthContext.Provider value={{ user, session, isLoading }}>
      {children}
    </AuthContext.Provider>
  )
}

export const useAuth = () => useContext(AuthContext)

Protected Routes (Next.js Middleware)

// src/middleware.ts
import { createMiddlewareClient } from '@supabase/auth-helpers-nextjs'
import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'

export async function middleware(req: NextRequest) {
  const res = NextResponse.next()
  const supabase = createMiddlewareClient({ req, res })
  const { data: { session } } = await supabase.auth.getSession()

  // Protected routes
  if (!session && req.nextUrl.pathname.startsWith('/dashboard')) {
    return NextResponse.redirect(new URL('/login', req.url))
  }

  // Redirect logged-in users from auth pages
  if (session && (req.nextUrl.pathname === '/login' || req.nextUrl.pathname === '/register')) {
    return NextResponse.redirect(new URL('/dashboard', req.url))
  }

  return res
}

export const config = {
  matcher: ['/dashboard/:path*', '/login', '/register']
}