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/index.html

@@ -62,6 +62,7 @@
         <div id="menu-popup" class="hidden">
           <button onclick="openSettings()">Settings</button>
           <!-- You can add more options here -->
+          <button id="bigpicture-btn" title="Launch Big Picture Mode (Controller/Steam Deck UI)">🎮 Big Picture Mode</button>
           <button id="devtools-btn" title="Open / Close Developer Tools">Toggle Developer Tools</button>
           <div class="zoom-controls">
             <button id="zoom-out-btn">-</button>