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

db/queries/users.sql

@@ -35,6 +35,9 @@ SELECT EXISTS(
     SELECT 1 FROM admin_permissions WHERE user_id = $1 AND permission = $2
 ) AS has_permission;
 
+-- name: CountUsers :one
+SELECT COUNT(*) FROM users;
+
 -- name: SearchUsersByEmailPrefix :many
 SELECT id, email FROM users WHERE email LIKE $1 || '%' ORDER BY email LIMIT 10;