Enhance theme management and add table canvas support

- Introduced new helper functions for managing table canvases in the GUI, allowing for better sizing and layout of elements.
- Updated the theme manager to include additional color properties for navigation elements, tree lines, and tab variations, improving visual consistency.
- Enhanced the theme selection interface with a more organized table layout for built-in and file themes, along with action buttons for theme management.

src/app/gui/theme_manager.h

@@ -105,6 +105,13 @@ struct EnhancedTheme {
   Color plot_lines_hovered;
   Color plot_histogram;
   Color plot_histogram_hovered;
+  Color tree_lines;
+  
+  // Additional ImGui colors for complete coverage
+  Color tab_dimmed;
+  Color tab_dimmed_selected;
+  Color tab_dimmed_selected_overline;
+  Color tab_selected_overline;
   
   // Style parameters
   float window_rounding = 0.0f;