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/style.css

@@ -5236,6 +5236,21 @@ select.settings-input option {
     font-family: 'JetBrains Mono', monospace;
     font-size: 0.875rem;
     letter-spacing: 0.5px;
+    -webkit-user-select: text !important;
+    -moz-user-select: text !important;
+    -ms-user-select: text !important;
+    user-select: text !important;
+    pointer-events: auto !important;
+}
+
+/* Ensure all input fields allow text selection and paste */
+input[type="text"].uuid-input,
+#customUuidInput {
+    -webkit-user-select: text !important;
+    -moz-user-select: text !important;
+    -ms-user-select: text !important;
+    user-select: text !important;
+    pointer-events: auto !important;
 }
 
 .uuid-btn {
@@ -5623,6 +5638,12 @@ select.settings-input option {
     color: #ef4444;
 }
 
+.uuid-item-btn.switch:hover {
+    background: rgba(59, 130, 246, 0.2);
+    border-color: rgba(59, 130, 246, 0.4);
+    color: #3b82f6;
+}
+
 @media (max-width: 600px) {
     .uuid-modal-content {
         width: 95vw;