v2.3.6: fix UUID loss during launcher updates

Players were losing character data (inventory, armor, backpack) after
each launcher update because config.json corruption wiped the UUID
mapping. Same username, new UUID = server treats as new player.

Fix: UUIDs now stored in separate uuid-store.json that saveConfig()
can never touch. Added safety check to refuse destructive writes
when config file exists but loads empty. Includes 28 regression tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/managers/gameLauncher.js

@@ -250,6 +250,7 @@ async function launchGame(playerNameOverride = null, progressCallback, javaPathO
   }
 
   const uuid = getUuidForUser(playerName);
+  console.log(`[Launcher] UUID for "${playerName}": ${uuid} (verify this stays constant across launches)`);
 
   // Fetch tokens from auth server
   if (progressCallback) {