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/pages/site/page.tsx

@@ -1,4 +1,3 @@
-import { useEffect, useState } from "react";
 import { useSiteStore } from "@/managers/stores/useSiteStore";
 import { SiteDisclaimerScreen } from "@/components/site/SiteDisclaimerScreen";
 import { SiteWelcomeScreen } from "@/components/site/SiteWelcomeScreen";
@@ -7,39 +6,7 @@ import { SiteNamingScreen } from "@/components/site/SiteNamingScreen";
 import { SiteTransitionOverlay } from "@/components/site/SiteTransitionOverlay";
 import { SiteMobileBlocker } from "@/components/site/SiteMobileBlocker";
 import { SiteLayout } from "@/components/site/SiteLayout";
-
-/**
- * Check if user is on mobile device
- */
-function useIsMobile(): boolean {
-  const [isMobile, setIsMobile] = useState(false);
-
-  useEffect(() => {
-    const checkMobile = (): void => {
-      const userAgent = navigator.userAgent.toLowerCase();
-      const mobileKeywords = [
-        "android",
-        "webos",
-        "iphone",
-        "ipad",
-        "ipod",
-        "blackberry",
-        "windows phone",
-      ];
-      const isMobileDevice = mobileKeywords.some((keyword) =>
-        userAgent.includes(keyword),
-      );
-      const isSmallScreen = window.innerWidth < 768;
-      setIsMobile(isMobileDevice || isSmallScreen);
-    };
-
-    checkMobile();
-    window.addEventListener("resize", checkMobile);
-    return () => window.removeEventListener("resize", checkMobile);
-  }, []);
-
-  return isMobile;
-}
+import { useIsMobile } from "@/hooks/ui/useIsMobile";
 
 export function SitePage(): React.JSX.Element {
   const currentStep = useSiteStore((state) => state.currentStep);