refactor(ui): parse the scoped theme entry at the boundary; runtime-switch owns transient keys

The scoped theme entry is now parsed by one boundary parser with a
stated invariant instead of ad hoc typeof narrowing, and the runtime
keys that mean "no instance connected" live next to the code that
produces them so a new sentinel cannot miss the theme-storage guard.

packages/ui/src/lib/runtime-switch.ts

@@ -51,6 +51,17 @@ const normalizeRuntimeUrlKey = (value: string): string => {
   }
 };
 
+// Runtime keys that mean "no instance connected": the uninitialized default
+// (`normalizeRuntimeUrlKey` of an empty/unparseable base URL) and the mobile
+// disconnect state (`MobileApp` switches to it when the connection drops).
+// Per-instance client state (e.g. the scoped theme entry) must not be read
+// from or written under them.
+export const MOBILE_DISCONNECTED_RUNTIME_KEY = 'mobile-disconnected';
+const UNINITIALIZED_RUNTIME_KEY = 'url:default';
+
+export const isTransientRuntimeKey = (runtimeKey: string): boolean =>
+  runtimeKey === '' || runtimeKey === UNINITIALIZED_RUNTIME_KEY || runtimeKey === MOBILE_DISCONNECTED_RUNTIME_KEY;
+
 const readInjectedApiBaseUrl = (): string => {
   if (typeof window === 'undefined') return '';
   const injected = (window as typeof window & { __OPENCHAMBER_API_BASE_URL__?: string }).__OPENCHAMBER_API_BASE_URL__;