feat: single instance lock; auto-size window to primary display bounds

- requestSingleInstanceLock(): second launch focuses existing window and quits.
- Window width/height/x/y set from screen.getPrimaryDisplay().bounds so the
  app fills the built-in display exactly on launch, regardless of resolution.

src/main/index.ts

@@ -1,4 +1,4 @@
-import { app, BrowserWindow, ipcMain } from 'electron';
+import { app, BrowserWindow, ipcMain, screen } from 'electron';
 import * as path from 'path';
 import * as os from 'os';
 import { loadSeedConfig } from './config_loader';
@@ -18,9 +18,13 @@ async function createWindow() {
     cachedFullConfig = await fetchFullConfig(cachedSeed);
   }
 
+  const { bounds } = screen.getPrimaryDisplay();
+
   mainWindow = new BrowserWindow({
-    width: 1600,
-    height: 900,
+    width: bounds.width,
+    height: bounds.height,
+    x: bounds.x,
+    y: bounds.y,
     title: 'OSIT Aether Launcher (Native)',
     webPreferences: {
       preload: path.join(__dirname, '../preload/index.js'),
@@ -55,15 +59,28 @@ registerShellHandlers();
 registerFileHandlers();
 registerSystemHandlers();
 
-app.on('ready', createWindow);
+// Single instance lock — if another instance is already running, focus it and quit.
+const gotTheLock = app.requestSingleInstanceLock();
+if (!gotTheLock) {
+  app.quit();
+} else {
+  app.on('second-instance', () => {
+    if (mainWindow) {
+      if (mainWindow.isMinimized()) mainWindow.restore();
+      mainWindow.focus();
+    }
+  });
 
-app.on('window-all-closed', () => {
-  if (process.platform !== 'darwin') app.quit();
-});
+  app.on('ready', createWindow);
 
-app.on('activate', () => {
-  if (mainWindow === null) createWindow();
-});
+  app.on('window-all-closed', () => {
+    if (process.platform !== 'darwin') app.quit();
+  });
+
+  app.on('activate', () => {
+    if (mainWindow === null) createWindow();
+  });
+}
 
 ipcMain.handle('get-seed-config', async () => cachedSeed || await loadSeedConfig());
 ipcMain.handle('get-device-config', async () => cachedFullConfig);