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/mod.rs

@@ -1854,6 +1854,8 @@ pub enum ModalKind {
     /// means and what usually fixes it. Queued once per verdict; the status
     /// bar's ⚠ pill reopens it.
     GpuWarning,
+    /// Reference Manager help window (toolbar Help button).
+    XrefHelp,
 }
 
 /// A property group controlled by a layer state's restore mask.
@@ -2473,6 +2475,8 @@ pub enum Message {
     XrefManagerRefreshMenu,
     /// Toggle the Change Path dropdown menu.
     XrefManagerPathMenu,
+    /// Open the Reference Manager help window.
+    XrefHelpOpen,
     /// Close all palette dropdown menus (overlay dismissal).
     XrefManagerDismissMenus,
     /// Open the file picker for Select New Path (anchor entry).