feat: multi-round messaging, login logo, applicant seed user

- Communication hub now supports selecting multiple rounds when sending
  to Round Jury or Round Applicants (checkbox list instead of dropdown)
- Recipients are unioned across selected rounds with deduplication
- Recipient details grouped by round when multiple rounds selected
- Added MOPC logo above "Welcome back" on login page
- Added matt@letsbe.solutions as seeded APPLICANT account

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/(auth)/login/page.tsx

@@ -4,6 +4,7 @@ import { useState } from 'react'
 import type { Route } from 'next'
 import { useSearchParams, useRouter } from 'next/navigation'
 import { signIn } from 'next-auth/react'
+import Image from 'next/image'
 import Link from 'next/link'
 import { Button } from '@/components/ui/button'
 import { Input } from '@/components/ui/input'
@@ -165,6 +166,15 @@ export default function LoginPage() {
     <Card className="w-full max-w-md overflow-hidden">
       <div className="h-1 w-full bg-gradient-to-r from-brand-blue via-brand-teal to-brand-blue" />
       <CardHeader className="text-center">
+        <div className="flex justify-center mb-2">
+          <Image
+            src="/images/MOPC-blue-small.png"
+            alt="MOPC"
+            width={48}
+            height={48}
+            className="h-12 w-auto"
+          />
+        </div>
         <CardTitle className="text-2xl">Welcome back</CardTitle>
         <CardDescription>
           {mode === 'password'