feat(sprint1-step2): core economy TS + useEconomy hook (lazy calc) + 13 tests vitest

Frontend/src/hooks/useEconomy.ts

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+// useEconomy.ts — Hook React avec lazy calculation + localStorage
+// Pas de setInterval pour les gains passifs — tout est calculé au read
+
+import { useState, useCallback, useEffect } from "react";
+import {
+  GameState,
+  DEFAULT_STATE,
+  applyIdleGains,
+  applyClick,
+  buyGenerator,
+  applyPrestige,
+  canPrestige,
+  totalProductionPerSecond,
+  generatorCost,
+} from "../core/economy";
+
+const SAVE_KEY = "clickerz_state";
+
+function loadState(): GameState {
+  try {
+    const raw = localStorage.getItem(SAVE_KEY);
+    if (!raw) return { ...DEFAULT_STATE, lastTick: Date.now() };
+    const saved = JSON.parse(raw) as GameState;
+    // Appliquer les gains idle accumulés pendant l'absence
+    return applyIdleGains(saved, Date.now());
+  } catch {
+    return { ...DEFAULT_STATE, lastTick: Date.now() };
+  }
+}
+
+function saveState(state: GameState): void {
+  localStorage.setItem(SAVE_KEY, JSON.stringify(state));
+}
+
+export function useEconomy() {
+  const [state, setState] = useState<GameState>(loadState);
+
+  // Auto-save + tick UI toutes les secondes (pour rafraîchir l'affichage uniquement)
+  // La vraie valeur est calculée lazily dans totalProductionPerSecond
+  useEffect(() => {
+    const id = setInterval(() => {
+      setState((prev) => {
+        const updated = applyIdleGains(prev, Date.now());
+        saveState(updated);
+        return updated;
+      });
+    }, 1000);
+    return () => clearInterval(id);
+  }, []);
+
+  const click = useCallback(() => {
+    setState((prev) => {
+      const updated = applyClick(applyIdleGains(prev, Date.now()));
+      saveState(updated);
+      return updated;
+    });
+  }, []);
+
+  const buy = useCallback((genId: string) => {
+    setState((prev) => {
+      const withIdle = applyIdleGains(prev, Date.now());
+      const updated = buyGenerator(withIdle, genId);
+      if (!updated) return prev;
+      saveState(updated);
+      return updated;
+    });
+  }, []);
+
+  const prestige = useCallback(() => {
+    setState((prev) => {
+      if (!canPrestige(prev)) return prev;
+      const updated = applyPrestige(prev);
+      saveState(updated);
+      return updated;
+    });
+  }, []);
+
+  const reset = useCallback(() => {
+    const fresh = { ...DEFAULT_STATE, lastTick: Date.now() };
+    saveState(fresh);
+    setState(fresh);
+  }, []);
+
+  return {
+    state,
+    click,
+    buy,
+    prestige,
+    canPrestige: canPrestige(state),
+    productionPerSecond: totalProductionPerSecond(state),
+    generatorCost,
+  };
+}