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/site/dialogueIds.ts

@@ -0,0 +1,9 @@
+/**
+ * Dialogue manifest IDs used by the /site flow and the intro sequence.
+ * Defined once here so components don't hold magic strings.
+ */
+export const SITE_DIALOGUE_IDS = {
+  naming: "narrateur_intro_prenom",
+  transition: "narrateur_intro_apresprenom",
+  introOrder: "narrateur_ordreebike",
+} as const;

src/data/site/siteConfig.ts

@@ -1,8 +1,23 @@
+import type { CSSProperties } from "react";
+
+const BACKGROUND_IMAGE = "/assets/bg-site.png";
+
 export const SITE_CONFIG = {
-  backgroundImage: "/assets/bg-site.png",
-  forcedName: "Danyl",
+  backgroundImage: BACKGROUND_IMAGE,
+  presetPlayerName: "Danyl",
 } as const;
 
+/**
+ * Shared background style used by SiteLayout and SiteMobileBlocker.
+ */
+export const SITE_BACKGROUND_STYLE: CSSProperties = {
+  backgroundColor: "#87CEEB",
+  backgroundImage: `url(${BACKGROUND_IMAGE})`,
+  backgroundSize: "cover",
+  backgroundPosition: "center",
+  backgroundRepeat: "no-repeat",
+};
+
 export interface SiteCardConfig {
   id: string;
   label: string;
@@ -24,10 +39,10 @@ export const EXPERIENCE_CARDS: readonly SiteCardConfig[] = [
  * Cards for screen 2: "Quelle est votre situation ?"
  */
 export const SITUATION_CARDS: readonly SiteCardConfig[] = [
-  { id: "sit-refugie-climat", label: "Sans domicile fixe", disabled: true },
+  { id: "sit-sans-domicile", label: "Sans domicile fixe", disabled: true },
   { id: "sit-refugie-guerre", label: "Réfugié.e de guerre", disabled: true },
   {
-    id: "sit-sans-domicile",
+    id: "sit-refugie-climat",
     label: "Réfugié.e climatique",
     disabled: false,
   },