feat: Reference Manager help window with web shortcomings

The toolbar Help button was a disabled tip that could never be clicked. It now opens a modal window explaining the manager on the web build: read-only reference list, disabled Attach/Refresh/Change Path (no filesystem access or native file pickers, so no add/reload/repath and no previews), nested read-only rows, and saved-drawing requirement for relative paths.

Includes a Report-an-issue button opening the GitHub issues page and a Close button; wired as ModalKind::XrefHelp through the shared modal frame so dismissal works on native and web.

src/app/update/mod.rs

@@ -2395,6 +2395,10 @@ impl OpenCADStudio {
                 self.xref_manager.refresh_open = false;
                 Task::none()
             }
+            Message::XrefHelpOpen => {
+                self.active_modal = Some(super::ModalKind::XrefHelp);
+                Task::none()
+            }
             Message::XrefManagerDismissMenus => {
                 self.xref_manager.attach_open = false;
                 self.xref_manager.refresh_open = false;