feat: embed images (OLE2FRAME), print pipeline fixes, plot prefs, community fonts

- IMAGEEMBED: pack raster images into OLE2FRAME entities so a .dwg is
  self-contained; 'Embed Image' entry in the XREF attach menu; control/MCP
  op embed_image for external tools; dxf_name for OLE2FRAME queries.
- Scene: populate_images_from_document now rebuilds OLE2FRAME and PDF
  underlay models too (they used to vanish after any XREF op).
- Printing: add PRINTER_ENUM_NETWORK pass when listing printers; GDI
  spooler fallback (hayro raster at 300 DPI + StretchDIBits) for hosts
  whose PDF handler registers no print verb; DEVMODE orientation with a
  90-degree raster rotation fallback for v4 drivers; fix the fit-scale
  precedence bug that printed blank pages.
- PDF export dialogs on Windows use the async file dialog so the native
  overwrite confirmation no longer deadlocks the UI thread.
- Plot dialog: persist the chosen printer (saved on selection) and the
  picked plot window across sessions; page setups with no device no
  longer reset the output device.
- Community fonts: detect unresolvable .shx references on open and offer
  a download from the repository's fonts/ folder (contribution guide in
  fonts/README.md); custom font source URL setting for intranet font
  servers; downloaded fonts resolve from the per-user fonts directory.

src/io/pdf_export.rs

@@ -375,7 +375,7 @@ fn image_triangle_matrix(p: [[f32; 2]; 3], uv: [[f32; 2]; 3]) -> Option<[f32; 6]
 /// The parent comes from `iced::window::run`, keeping the portal request tied
 /// to the visible app window on Wayland instead of silently resolving to
 /// `None` on desktops that reject a parentless save dialog (#537).
-#[cfg(not(target_arch = "wasm32"))]
+#[cfg(all(not(target_arch = "wasm32"), not(target_os = "windows")))]
 pub fn pick_pdf_path_owned(
     stem: String,
     parent: &dyn iced::window::Window,
@@ -392,6 +392,25 @@ pub fn pick_pdf_path_owned(
     Some(path)
 }
 
+/// Windows: pick the PDF destination with the async dialog on a worker
+/// thread. The parented blocking dialog ran `IFileDialog::Show` on the UI
+/// thread inside the window callback, and when the target name already
+/// existed the overwrite-confirmation popup is a second nested modal that
+/// never gets pumped there — the app froze instead of asking. The async
+/// backend runs the dialog off-thread; it is the same pattern the DWG
+/// Save As flow uses, whose confirm popup works.
+#[cfg(all(not(target_arch = "wasm32"), target_os = "windows"))]
+pub async fn pick_pdf_path_async(stem: String) -> Option<std::path::PathBuf> {
+    let handle = crate::sys::file_dialog()
+        .set_title(crate::t!("Export as PDF").as_ref())
+        .set_file_name(format!("{stem}.pdf"))
+        .add_filter(crate::t!("PDF Files").as_ref(), &["pdf"])
+        .add_filter(crate::t!("All Files").as_ref(), &["*"])
+        .save_file()
+        .await?;
+    Some(crate::sys::handle_path(&handle))
+}
+
 // ── PDF builder ───────────────────────────────────────────────────────────
 
 #[cfg(not(target_arch = "wasm32"))]