fix: comprehensive UUID/username persistence bug fixes

Major fixes for UUID/skin reset issues that caused players to lose cosmetics:

Core fixes:
- Username rename now preserves UUID (atomic rename, not new identity)
- Atomic config writes with backup/recovery system
- Case-insensitive UUID lookup with case-preserving storage
- Pre-launch validation blocks play if no username configured
- Removed saveUsername calls from launch/install flows

UUID Modal fixes:
- Fixed isCurrent badge showing on wrong user
- Added switch identity button to change between saved usernames
- Fixed custom UUID input using unsaved DOM username
- UUID list now refreshes when player name changes
- Enabled copy/paste in custom UUID input field

UI/UX improvements:
- Added translation keys for switch username functionality
- CSS user-select fix for UUID input fields
- Allowed Ctrl+V/C/X/A shortcuts in Electron

Files: config.js, gameLauncher.js, gameManager.js, playerManager.js,
launcher.js, settings.js, main.js, preload.js, style.css, en.json

See UUID_BUGS_FIX_PLAN.md for detailed bug list (18 bugs, 16 fixed)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/launcher.js

@@ -5,6 +5,8 @@
 const {
   saveUsername,
   loadUsername,
+  loadUsernameWithDefault,
+  hasUsername,
   saveJavaPath,
   loadJavaPath,
   saveInstallPath,
@@ -27,9 +29,11 @@ const {
   getUuidForUser,
   isFirstLaunch,
   markAsLaunched,
+  checkLaunchReady,
   // UUID Management
   getCurrentUuid,
   getAllUuidMappings,
+  getAllUuidMappingsArray,
   setUuidForUser,
   generateNewUuid,
   deleteUuidForUser,
@@ -110,7 +114,10 @@ module.exports = {
   // User configuration functions
   saveUsername,
   loadUsername,
+  loadUsernameWithDefault,
+  hasUsername,
   getUuidForUser,
+  checkLaunchReady,
 
   // Java configuration functions
   saveJavaPath,
@@ -162,6 +169,7 @@ module.exports = {
   // UUID Management functions
   getCurrentUuid,
   getAllUuidMappings,
+  getAllUuidMappingsArray,
   setUuidForUser,
   generateNewUuid,
   deleteUuidForUser,