Refactor Big Picture Mode to use main window

Big Picture Mode now launches in the main application window instead of a separate Electron window. This reduces resource usage, prevents SteamOS desktop mode conflicts, and enables seamless switching between Desktop and Big Picture modes. Updated documentation, main process logic, and preload API to reflect the new architecture.

preload.js

@@ -442,14 +442,18 @@ contextBridge.exposeInMainWorld('aboutAPI', {
 });
 
 // Big Picture Mode API - Steam Deck / Console UI
+// Note: Big Picture Mode now opens in the main window (not a separate window) to keep resources low
+// and prevent SteamOS from creating desktop mode alongside when auto-launching.
 contextBridge.exposeInMainWorld('bigPictureAPI', {
   // Get screen info to determine if Big Picture Mode is recommended
   getScreenInfo: () => ipcRenderer.invoke('get-screen-info'),
   // Check if device is likely a Steam Deck or handheld
   isSuggested: () => ipcRenderer.invoke('is-bigpicture-suggested'),
-  // Launch Big Picture Mode
+  // Check if currently in Big Picture Mode
+  isActive: () => ipcRenderer.invoke('is-in-bigpicture'),
+  // Launch Big Picture Mode (navigates main window to Big Picture UI)
   launch: () => ipcRenderer.invoke('launch-bigpicture'),
-  // Exit Big Picture Mode
+  // Exit Big Picture Mode (navigates main window back to desktop UI)
   exit: () => ipcRenderer.invoke('exit-bigpicture'),
   // Navigate to URL (from Big Picture Mode)
   navigate: (url) => ipcRenderer.send('bigpicture-navigate', url),