v2.0.5: Fix E2E flow - proxy, welcome screen, provider sync

Critical fixes:
- Translation proxy now uses system Node.js (not Electron binary)
- Removed duplicate proxy start causing port conflicts
- Added port availability check before spawning proxy
- Fixed welcome:choice double resolve()
- Fixed settings.html close using deprecated remote
- Fixed translationProxy /v1 for openai-compat backends
- Proxy no longer detached/unref - properly tracked as child
- SingletonLock cleanup on startup

Verified E2E:
- Welcome screen on first run ✓
- Provider selection works ✓
- Settings save + sync ✓
- Translation proxy starts correctly ✓
- LS connects to proxy ✓
- --ag-reset works ✓

dist/provider/settings.html

@@ -585,9 +585,7 @@ async function saveSettings() {
       updateStatus();
       // Auto-close settings window after a brief delay
       setTimeout(() => {
-        const currentWindow = require('electron').remote?.getCurrentWindow();
-        if (currentWindow) currentWindow.close();
-        // Fallback: close via ipcRenderer
+        // Close via IPC — providerSettings.js handles the actual close
         require('electron').ipcRenderer.send('provider:close-settings');
       }, 1500);
     } else {