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.

renderer/script.js

@@ -1189,6 +1189,20 @@ window.addEventListener('DOMContentLoaded', () => {
     });
   }
 
+  // Big Picture Mode button
+  const bigPictureBtn = document.getElementById('bigpicture-btn');
+  if (bigPictureBtn && window.bigPictureAPI && window.bigPictureAPI.launch) {
+    bigPictureBtn.addEventListener('click', async () => {
+      try {
+        await window.bigPictureAPI.launch();
+        // Close the menu popup
+        if (menuPopup) menuPopup.classList.add('hidden');
+      } catch (e) {
+        console.error('Failed to launch Big Picture Mode:', e);
+      }
+    });
+  }
+
   // wire up back/forward buttons
   const backBtn = document.querySelector('.nav-left button:nth-child(1)');
   const forwardBtn = document.querySelector('.nav-left button:nth-child(2)');