Pool page: add bulk assign-to-round, enhance project pool UI

- Add assignAllToRound mutation to project-pool router
- Rewrite pool page with round selector, bulk assignment, and better layout
- Add pool navigation link to admin projects page

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

src/app/(admin)/admin/projects/page.tsx

@@ -622,6 +622,12 @@ export default function ProjectsPage() {
             <Bot className="mr-2 h-4 w-4" />
             AI Tags
           </Button>
+          <Button variant="outline" asChild>
+            <Link href="/admin/projects/pool">
+              <Layers className="mr-2 h-4 w-4" />
+              Assign to Round
+            </Link>
+          </Button>
           <Button variant="outline" asChild>
             <Link href="/admin/projects/bulk-upload">
               <FileUp className="mr-2 h-4 w-4" />