feat: buy x1/x5/x10/xMax + production preview per generator

- buyGenerator() supports quantity param (multi-buy loop)
- maxAffordable() / bulkCost() — compute max purchasable + total cost
- GeneratorShop: mode selector (x1/x5/x10/MAX)
- Each generator shows +X/s in amber — what the next purchase adds
- Button shows total cost + quantity (e.g. "1.5k (x5)")

Frontend/src/lib/core/economy.ts

@@ -661,22 +661,62 @@ export function applyClick(state: GameState, rng: number = Math.random()): Click
 }
 
 // Achat d'un générateur (retourne null si fonds insuffisants)
-export function buyGenerator(state: GameState, genId: string): GameState | null {
+export function buyGenerator(state: GameState, genId: string, quantity = 1): GameState | null {
   const genIndex = state.generators.findIndex((g) => g.id === genId);
   if (genIndex === -1) return null;
 
-  const gen = state.generators[genIndex];
-  const cost = generatorCost(gen, state.evolutionTree);
-  if (state.resources < cost) return null;
+  let gen = { ...state.generators[genIndex] };
+  let resources = state.resources;
+
+  let bought = 0;
+  for (let i = 0; i < quantity; i++) {
+    const cost = generatorCost(gen, state.evolutionTree);
+    if (resources < cost) break;
+    resources -= cost;
+    gen = { ...gen, owned: gen.owned + 1 };
+    bought++;
+  }
+
+  if (bought === 0) return null;
 
   const updatedGenerators = [...state.generators];
-  updatedGenerators[genIndex] = { ...gen, owned: gen.owned + 1 };
+  updatedGenerators[genIndex] = gen;
 
-  return {
-    ...state,
-    resources: state.resources - cost,
-    generators: updatedGenerators,
-  };
+  return { ...state, resources, generators: updatedGenerators };
+}
+
+// Calcule combien d'unités on peut acheter avec les ressources actuelles
+export function maxAffordable(state: GameState, genId: string): number {
+  const genIndex = state.generators.findIndex((g) => g.id === genId);
+  if (genIndex === -1) return 0;
+
+  let gen = { ...state.generators[genIndex] };
+  let resources = state.resources;
+  let count = 0;
+
+  while (true) {
+    const cost = generatorCost(gen, state.evolutionTree);
+    if (resources < cost) break;
+    resources -= cost;
+    gen = { ...gen, owned: gen.owned + 1 };
+    count++;
+    if (count > 1000) break; // safety
+  }
+  return count;
+}
+
+// Cout total pour acheter N unités
+export function bulkCost(state: GameState, genId: string, quantity: number): number {
+  const genIndex = state.generators.findIndex((g) => g.id === genId);
+  if (genIndex === -1) return Infinity;
+
+  let gen = { ...state.generators[genIndex] };
+  let total = 0;
+  for (let i = 0; i < quantity; i++) {
+    total += generatorCost(gen, state.evolutionTree);
+    gen = { ...gen, owned: gen.owned + 1 };
+  }
+  return total;
 }
 
 // Prestige : reset run, gain ADN, arbre persiste