feat: Add natural D-pad navigation for hub sections

- Add left navigation from hubs to sidebar when at leftmost item
- Add up navigation from first hub to tab bar (Libraries) or hero section (Discover)
- Enhance HubSection with onNavigateLeft and onNavigateUp callbacks
- Update _handleVerticalNavigation to return false at top boundary for proper UP handling
- Implement _navigateToSidebar() in Library Recommended tab and Discover screen
- Update all HubSection instances with proper navigation callbacks

lib/widgets/hub_section.dart

@@ -39,6 +39,14 @@ class HubSection extends StatefulWidget {
   /// Used to navigate focus back to the tab bar.
   final VoidCallback? onBack;
 
+  /// Called when the user presses LEFT while at the leftmost item.
+  /// Used to navigate focus to the side navigation rail.
+  final VoidCallback? onNavigateLeft;
+
+  /// Called when the user presses UP while at the topmost item (first hub).
+  /// Used to navigate focus to the tab bar.
+  final VoidCallback? onNavigateUp;
+
   const HubSection({
     super.key,
     required this.hub,
@@ -49,6 +57,8 @@ class HubSection extends StatefulWidget {
     this.showServerName = false,
     this.onVerticalNavigation,
     this.onBack,
+    this.onNavigateLeft,
+    this.onNavigateUp,
   });
 
   @override
@@ -172,6 +182,9 @@ class HubSectionState extends State<HubSection> {
         HubFocusMemory.setForHub(widget.hub.hubKey, _focusedIndex);
         _scrollToIndex(_focusedIndex);
         setState(() {});
+      } else if (widget.onNavigateLeft != null) {
+        // At leftmost item - navigate to sidebar
+        widget.onNavigateLeft!();
       }
       return KeyEventResult.handled;
     }
@@ -189,7 +202,11 @@ class HubSectionState extends State<HubSection> {
 
     // Up/Down: delegate to parent for vertical hub navigation, ALWAYS consume
     if (key.isUpKey) {
-      widget.onVerticalNavigation?.call(true);
+      final handled = widget.onVerticalNavigation?.call(true) ?? false;
+      // If not handled (at top boundary) and we have onNavigateUp, call it
+      if (!handled && widget.onNavigateUp != null) {
+        widget.onNavigateUp!();
+      }
       return KeyEventResult.handled;
     }
     if (key.isDownKey) {