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).

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;

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