fix: always merge evolutionTree/generators with defaults on load

Server saves don't store all node fields (branch, cost, effect, etc.).
migrateSave now always rebuilds tree from DEFAULT_EVOLUTION_TREE,
preserving unlocked state from the save. Fixes empty evolution tree.

Frontend/src/lib/core/migrateSave.ts

@@ -23,6 +23,15 @@ export function migrateSave(raw: Record<string, unknown>): GameState {
   // Futures migrations :
   // if (version < 3) state = migrateV2toV3(state);
 
+  // Always rebuild tree & generators from defaults — the server/localStorage
+  // may not store all fields (branch, cost, effect, baseProduction, etc.)
+  state.evolutionTree = mergeEvolutionTree(
+    state.evolutionTree as Array<Record<string, unknown>> | undefined
+  );
+  state.generators = mergeGenerators(
+    state.generators as Array<Record<string, unknown>> | undefined
+  );
+
   return state as unknown as GameState;
 }