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>

GUI/locales/en.json

@@ -211,7 +211,11 @@
     "modsDeleteFailed": "Failed to delete mod: {error}",
     "modsModNotFound": "Mod information not found",
     "hwAccelSaved": "Hardware acceleration setting saved",
-    "hwAccelSaveFailed": "Failed to save hardware acceleration setting"
+    "hwAccelSaveFailed": "Failed to save hardware acceleration setting",
+    "noUsername": "No username configured. Please save your username first.",
+    "switchUsernameSuccess": "Switched to \"{username}\" successfully!",
+    "switchUsernameFailed": "Failed to switch username",
+    "playerNameTooLong": "Player name must be 16 characters or less"
   },
   "confirm": {
     "defaultTitle": "Confirm action",
@@ -226,7 +230,10 @@
     "deleteUuidButton": "Delete",
     "uninstallGameTitle": "Uninstall game",
     "uninstallGameMessage": "Are you sure you want to uninstall Hytale? All game files will be deleted.",
-    "uninstallGameButton": "Uninstall"
+    "uninstallGameButton": "Uninstall",
+    "switchUsernameTitle": "Switch Identity",
+    "switchUsernameMessage": "Switch to username \"{username}\"? This will change your current player identity.",
+    "switchUsernameButton": "Switch"
   },
   "progress": {
     "initializing": "Initializing...",