Multiple admins

Adds Player.is_admin (creator starts with it; backfilled by migration).
Admin-key-authenticated POST /admin/set-admin grants/revokes it from the
admin panel; the creator's privileges can't be revoked. Admin-gated
backend routes (dev mode, skip character creation) and frontend controls
(corner menu, lobby start, scene start, end story) now check is_admin
instead of is_creator, and admins get the admin_key in their state blob
so granted players can open the admin panel.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/pages/Dashboard.svelte

@@ -94,9 +94,14 @@
         }
     }
 
+    // Admins get the admin key in their state blob; stash it so the Admin page works for them too
+    $: if (state?.player?.is_admin && state?.game?.admin_key) {
+        localStorage.setItem(`admin_key_${gameId}`, state.game.admin_key);
+    }
+
     $: {
         const items = [];
-        if (state?.player?.is_creator) {
+        if (state?.player?.is_admin) {
             items.push({
                 label: `🧪 Dev Mode: ${state.game.dev_mode ? 'On' : 'Off'}`,
                 action: toggleDevMode,