Beautify UI across all tabs, restructure shells to multi-file, polish unit dialog

- Reorder tabs: Rex Config, Units, Plans, Shells
- Beautify Units, Plans, Shells, and Config tabs with framed lists,
  linked button groups, consistent margins, and dim labels
- Grey out plan controls and task properties when no plan/task loaded
- Restructure shells from single-file to multi-file directory model,
  paralleling the units architecture (create/delete/save files,
  create/delete/edit/move shells within files)
- Fix shells loading to scan directories for .shells files
- Beautify unit properties dialog with GtkFrame sections, dim-label
  field labels, internal padding, and linked file action buttons
- Add rectifier Select/Open/Create file buttons
- Fix GtkSwitch multi-click issue using state-set signal
- Move Save File button to unit files sidebar
- Sync unit editor state after name changes in properties dialog

src/views/plan_view.h

@@ -44,6 +44,7 @@ private:
     GtkWidget* btn_create_plan_;
     GtkWidget* btn_close_plan_;
     GtkWidget* btn_save_plan_;
+    GtkWidget* task_controls_;   // container for task list + buttons
     UnitEditor* unit_editor_;
 
     int current_task_idx_ = -1;