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/models/project.h

@@ -37,7 +37,7 @@ public:
 
     std::vector<Plan> plans;
     std::vector<UnitFile> unit_files;
-    ShellsFile shells;
+    std::vector<ShellsFile> shell_files;
 
     // status reporting
     using StatusCallback = void(*)(const std::string& msg, void* data);
@@ -55,11 +55,10 @@ public:
     bool is_unit_name_taken(const std::string& name, const Unit* exclude = nullptr) const;
     void load_all_units();
     void load_plan(const std::filesystem::path& plan_path);
-    void load_shells(const std::filesystem::path& shells_path);
     void reload_shells();
+    std::vector<ShellDef> all_shells() const;
     void save_config();
     void save_plans();
-    void save_shells();
 
     void open_config(const std::filesystem::path& new_config_path);
     void close_config();