feat(scope): player store scopedBotId + resolveScopedBot helper

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/stores/scope.ts

@@ -0,0 +1,10 @@
+/** Given the desired scoped id (from ?bot) and the known bot ids, decide the
+ * effective scope. Returns the id if it exists, else null (graceful clear:
+ * a stale/forbidden id never locks the UI). */
+export function resolveScopedBot(
+  requestedId: string | null | undefined,
+  knownBotIds: readonly string[],
+): string | null {
+  if (!requestedId) return null;
+  return knownBotIds.includes(requestedId) ? requestedId : null;
+}