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/view/modal.rs

@@ -56,6 +56,7 @@ impl OpenCADStudio {
             Some(K::MissingFonts) => crate::t!("Missing fonts").into_owned(),
             Some(K::RecoveryPrompt) => crate::tr!("modal", "recovery-prompt"),
             Some(K::GpuWarning) => crate::tr!("gpu", "title"),
+            Some(K::XrefHelp) => crate::t!("Reference Manager Help").into_owned(),
             None => String::new(),
         }
     }
@@ -85,6 +86,9 @@ impl OpenCADStudio {
             super::super::ModalKind::About => {
                 automatic_flow(ex, crate::ui::window::about::view_window)
             }
+            super::super::ModalKind::XrefHelp => {
+                automatic_flow(ex, crate::ui::window::xref_help::view_window)
+            }
             super::super::ModalKind::Shortcuts => {
                 // Keys claimed by two rows — the cells turn red and a
                 // persistent warning names the command already using each