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

internal/db/users.sql.go

@@ -11,6 +11,17 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
+const countUsers = `-- name: CountUsers :one
+SELECT COUNT(*) FROM users
+`
+
+func (q *Queries) CountUsers(ctx context.Context) (int64, error) {
+	row := q.db.QueryRow(ctx, countUsers)
+	var count int64
+	err := row.Scan(&count)
+	return count, err
+}
+
 const deleteAdminPermission = `-- name: DeleteAdminPermission :exec
 DELETE FROM admin_permissions WHERE user_id = $1 AND permission = $2
 `