feat: add login page and auth callback route

- Replace default Next.js home with role-aware root redirect
- Add email/password login form (client component, signInWithPassword)
- Add auth callback route for Supabase code exchange
- Login page at /login, callback at /api/auth/callback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AWxyMibCuGGtSQSqfajDQ7

app/api/auth/callback/route.ts

@@ -0,0 +1,19 @@
+// app/api/auth/callback/route.ts
+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') ?? '/'
+
+  if (code) {
+    const supabase = await createClient()
+    const { error } = await supabase.auth.exchangeCodeForSession(code)
+    if (!error) {
+      return NextResponse.redirect(`${origin}${next}`)
+    }
+  }
+
+  return NextResponse.redirect(`${origin}/login?error=auth_callback_failed`)
+}