Add admin capsule management, fix file browser for special files, normalize dialog styles

- Admin capsule CRUD: list, create (platform templates), get detail with
  terminal/files/metrics, snapshot, destroy
- First signup auto-promotes to platform admin
- JWT auth via query param for WebSocket connections
- File browser: handle non-regular files (devices, pipes, sockets) gracefully
  instead of showing raw backend errors
- Normalize admin template dialogs to match established dialog patterns:
  remove accent bars, unify animation/shadow/button styles

frontend/src/lib/components/AdminSidebar.svelte

@@ -3,7 +3,8 @@
 	import { auth } from '$lib/auth.svelte';
 	import {
 		IconServer,
-		IconTemplate,
+		IconBox,
+		IconMonitor,
 		IconSettings,
 		IconLogout,
 		IconSidebar,
@@ -22,7 +23,8 @@
 	};
 
 	const managementItems: NavItem[] = [
-		{ label: 'Templates', icon: IconTemplate, href: '/admin/templates' },
+		{ label: 'Templates', icon: IconBox, href: '/admin/templates' },
+		{ label: 'Capsules', icon: IconMonitor, href: '/admin/capsules' },
 		{ label: 'Hosts', icon: IconServer, href: '/admin/hosts' }
 	];