refactor(site): extract shared utilities and centralise dialogue IDs

- new src/hooks/ui/useIsMobile.ts (matchMedia + useSyncExternalStore)
  replacing the resize-handler hook inlined inside pages/site/page.tsx
- new src/hooks/ui/usePrefersReducedMotion.ts
- new src/data/site/dialogueIds.ts so site and intro components stop
  carrying hard-coded narrator IDs
- siteConfig: add SITE_BACKGROUND_STYLE shared by SiteLayout and
  SiteMobileBlocker, rename forcedName to presetPlayerName, fix the
  swapped id/label pairing on situation cards
- useSiteStore: rename selectedExperience/Situation to *Index so the
  stored value (an array index) is obvious in callers
- audioConfig: drop dead AUDIO_PATHS placeholders
- propagate the renames and SITE_BACKGROUND_STYLE through SiteLayout,
  SiteWelcomeScreen, SiteSituationScreen and pages/site/page.tsx

src/hooks/ui/usePrefersReducedMotion.ts

@@ -0,0 +1,29 @@
+import { useSyncExternalStore } from "react";
+
+const REDUCED_MOTION_QUERY = "(prefers-reduced-motion: reduce)";
+
+function subscribeToReducedMotion(callback: () => void): () => void {
+  const query = window.matchMedia(REDUCED_MOTION_QUERY);
+  query.addEventListener("change", callback);
+  return () => query.removeEventListener("change", callback);
+}
+
+function getReducedMotionSnapshot(): boolean {
+  return window.matchMedia(REDUCED_MOTION_QUERY).matches;
+}
+
+function getServerReducedMotionSnapshot(): boolean {
+  return false;
+}
+
+/**
+ * True when the user has requested reduced motion at the OS level.
+ * UI fades and transitions should collapse to 0ms when this is true.
+ */
+export function usePrefersReducedMotion(): boolean {
+  return useSyncExternalStore(
+    subscribeToReducedMotion,
+    getReducedMotionSnapshot,
+    getServerReducedMotionSnapshot,
+  );
+}