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/settings.html

@@ -30,6 +30,18 @@
         </div>
         <p class="note">Settings are stored locally on this device.</p>
 
+        <!-- Big Picture Mode -->
+        <div class="setting-group">
+          <h3>🎮 Big Picture Mode</h3>
+          <p class="note">A controller-friendly UI designed for Steam Deck and handheld devices.</p>
+          <div style="display: flex; align-items: center; gap: 12px; margin-top: 10px;">
+            <button id="launch-bigpicture-btn" class="primary-btn">
+              <span style="font-size: 18px;">🎮</span> Launch Big Picture Mode
+            </button>
+            <span id="bigpicture-status" class="note"></span>
+          </div>
+        </div>
+
         <div class="setting-group">
           <h3>Weather</h3>
           <fieldset class="weather-units" style="display:flex; flex-direction:column; gap:8px; border:1px solid rgba(255,255,255,0.15); padding:10px; border-radius:8px;">