feat(01-03): add auth callback route for code exchange

- Handle OAuth and email verification callback
- Exchange code for session using Supabase SSR
- Redirect to next param or /dashboard on success
- Redirect to /login with error on failure

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

src/app/auth/callback/route.ts

@@ -0,0 +1,20 @@
+import { createClient } from '@/lib/supabase/server'
+import { NextResponse } from 'next/server'
+
+export async function GET(request: Request) {
+  const { searchParams, origin } = new URL(request.url)
+  const code = searchParams.get('code')
+  const next = searchParams.get('next') ?? '/dashboard'
+
+  if (code) {
+    const supabase = await createClient()
+    const { error } = await supabase.auth.exchangeCodeForSession(code)
+
+    if (!error) {
+      return NextResponse.redirect(`${origin}${next}`)
+    }
+  }
+
+  // Return the user to an error page with instructions
+  return NextResponse.redirect(`${origin}/login?error=auth_callback_error`)
+}