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/data/audioConfig.ts

@@ -1,11 +1,3 @@
-export const AUDIO_PATHS = {
-  intro: "/sounds/effect/fa.mp3",
-  bienvenue: "/sounds/effect/fa.mp3",
-  alertCentral: "/sounds/effect/fa.mp3",
-  searching: "/sounds/effect/fa.mp3",
-  helped: "/sounds/effect/fa.mp3",
-} as const;
-
 export type AudioCategory = "music" | "sfx" | "dialogue";
 
 export const DEFAULT_CATEGORY_VOLUMES: Record<AudioCategory, number> = {