Add Big Picture Mode for Steam Deck and controllers

Introduces a new Big Picture Mode with a controller-friendly, console-style UI optimized for Steam Deck and handheld devices. Adds new renderer files (HTML, CSS, JS) for the mode, updates main and preload scripts to support window management and IPC for Big Picture Mode, and documents features and usage in BIG_PICTURE_MODE.md. Updates settings and navigation to allow launching and exiting Big Picture Mode.

preload.js

@@ -116,6 +116,20 @@ contextBridge.exposeInMainWorld('aboutAPI', {
   getInfo: () => ipcRenderer.invoke('get-about-info')
 });
 
+// Big Picture Mode API - Steam Deck / Console UI
+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
+  launch: () => ipcRenderer.invoke('launch-bigpicture'),
+  // Exit Big Picture Mode
+  exit: () => ipcRenderer.invoke('exit-bigpicture'),
+  // Navigate to URL (from Big Picture Mode)
+  navigate: (url) => ipcRenderer.send('bigpicture-navigate', url)
+});
+
 // Relay context-menu commands from main to active renderer context (open new tabs etc.)
 ipcRenderer.on('context-menu-command', (event, payload) => {
   window.dispatchEvent(new CustomEvent('nebula-context-command', { detail: payload }));