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.css

@@ -217,6 +217,35 @@ button:active {
   transform: translateY(1px);
 }
 
+/* Primary button style (e.g., Big Picture Mode) */
+.primary-btn {
+  display: inline-flex;
+  align-items: center;
+  gap: 8px;
+  padding: 12px 24px;
+  font-size: 1rem;
+  font-weight: 600;
+  background: linear-gradient(135deg, #7B2EFF 0%, #5a1fd4 50%, #00C6FF 100%);
+  background-size: 200% 100%;
+  color: #fff;
+  border: 1px solid rgba(255,255,255,0.15);
+  border-radius: 12px;
+  cursor: pointer;
+  transition: all 0.3s ease;
+  box-shadow: 0 4px 20px rgba(123, 46, 255, 0.3);
+}
+
+.primary-btn:hover {
+  background-position: 100% 0;
+  box-shadow: 0 6px 30px rgba(123, 46, 255, 0.45), 0 0 20px rgba(0, 198, 255, 0.2);
+  transform: translateY(-2px);
+}
+
+.primary-btn:active {
+  transform: translateY(0);
+  box-shadow: 0 2px 10px rgba(123, 46, 255, 0.3);
+}
+
 .note {
   font-size: 0.8rem;
   color: #aaa;