Add sidebar toggle and fullscreen webview support

Implements a sidebar hidden state and toggle functionality for fullscreen webview mode, including new CSS for sidebar transitions and a menu hint. Updates gamepad input handling to allow toggling the sidebar and scrolling in webview mode, and restricts focusable elements to the sidebar and header when browsing. Improves user experience when navigating and interacting with web content in fullscreen.

renderer/bigpicture.css

@@ -1426,3 +1426,54 @@ body.fullscreen .bp-main {
   font-size: 0.7rem;
   color: var(--bp-text);
 }
+
+/* =============================================================================
+   SIDEBAR HIDDEN STATE (Fullscreen webview mode)
+   ============================================================================= */
+
+.bp-sidebar.sidebar-hidden {
+  transform: translateX(-100%);
+  opacity: 0;
+  pointer-events: none;
+  width: 0;
+  min-width: 0;
+  padding: 0;
+  border: none;
+  transition: all 0.3s ease-out;
+}
+
+.bp-sidebar {
+  transition: all 0.3s ease-out;
+}
+
+.bp-content.fullscreen {
+  margin-left: 0;
+  width: 100%;
+}
+
+.bp-header.sidebar-hidden .header-left {
+  opacity: 0.5;
+  transform: scale(0.9);
+  transition: all 0.3s ease;
+}
+
+/* Show sidebar toggle hint when in fullscreen */
+.bp-content.fullscreen::before {
+  content: '☰ Menu';
+  position: fixed;
+  bottom: 20px;
+  left: 20px;
+  background: rgba(20, 20, 31, 0.8);
+  border: 1px solid var(--bp-border);
+  border-radius: var(--bp-radius-sm);
+  padding: 8px 12px;
+  font-size: 0.75rem;
+  color: var(--bp-text-muted);
+  z-index: 100;
+  opacity: 0.6;
+  transition: opacity 0.2s ease;
+}
+
+.bp-content.fullscreen:hover::before {
+  opacity: 1;
+}