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/ui/window/mod.rs

@@ -22,4 +22,5 @@ pub mod annotation_data;
 pub mod alias_editor;
 pub mod find_replace;
 pub mod named_parameters;
+pub mod xref_help;
 pub mod xref_manager;