feat: click upgrades — buy click power with tadpoles, tied to generators

5 click upgrades, each linked to a generator type:
- Nid Douillet (+1/clic, 50 base) — requires owning a Nid
- Eau Fertile (+3/clic, 500 base) — requires a Mare
- Spores Actives (+8/clic, 5k base) — requires a Marecage
- Courant Vital (+20/clic, 50k base) — requires an Etang
- Source Ancestrale (+50/clic, 500k base) — requires a Lac

Cost scales x1.2 per level. Reset at prestige (like generators).
Click gain = (base + upgradePower) × prestige × tree × infraBonus.
ClickPanel shows upgrade shop with level badges and gen requirements.
Adds tadpole sink for active play — strategic choice vs buying generators.

Frontend/src/lib/core/migrateSave.ts

@@ -3,8 +3,8 @@
 // Chaque sprint ajoute un step (v2→v3, etc.)
 
 import { CURRENT_SAVE_VERSION } from "./balance";
-import type { GameState } from "./economy";
-import { DEFAULT_EVOLUTION_TREE, DEFAULT_GENERATORS } from "./economy";
+import type { GameState, ClickUpgrade } from "./economy";
+import { DEFAULT_EVOLUTION_TREE, DEFAULT_GENERATORS, DEFAULT_CLICK_UPGRADES } from "./economy";
 
 /**
  * Détecte la version d'une save et applique les migrations nécessaires.
@@ -32,6 +32,11 @@ export function migrateSave(raw: Record<string, unknown>): GameState {
     state.generators as Array<Record<string, unknown>> | undefined
   );
 
+  // Click upgrades — merge with defaults (preserves levels, adds new upgrades)
+  state.clickUpgrades = mergeClickUpgrades(
+    state.clickUpgrades as Array<Record<string, unknown>> | undefined
+  );
+
   return state as unknown as GameState;
 }
 
@@ -149,3 +154,25 @@ function mergeGenerators(
     return { ...defaultGen };
   });
 }
+
+/**
+ * Merge les click upgrades sauvegardés avec DEFAULT_CLICK_UPGRADES.
+ * Conserve le level, met à jour les stats de base.
+ */
+function mergeClickUpgrades(
+  saved: Array<Record<string, unknown>> | undefined
+): ClickUpgrade[] {
+  if (!saved || !Array.isArray(saved)) {
+    return DEFAULT_CLICK_UPGRADES.map((u) => ({ ...u }));
+  }
+
+  const savedById = new Map(saved.map((u) => [u.id as string, u]));
+
+  return DEFAULT_CLICK_UPGRADES.map((def) => {
+    const s = savedById.get(def.id);
+    if (s) {
+      return { ...def, level: typeof s.level === "number" ? s.level : 0 };
+    }
+    return { ...def };
+  });
+}