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/scene/entity.rs

@@ -2153,19 +2153,19 @@ impl Scene {
 
     fn populate_images_from_document_unbumped(&mut self) {
         self.images.clear();
-        let entries: Vec<(Handle, acadrust::entities::RasterImage)> = self
+        let entries: Vec<(Handle, acadrust::entities::EntityType)> = self
             .document
             .entities()
-            .filter_map(|e| {
-                if let EntityType::RasterImage(img) = e {
-                    Some((img.common.handle, img.clone()))
-                } else {
-                    None
-                }
+            .filter(|e| {
+                matches!(
+                    e,
+                    EntityType::RasterImage(_) | EntityType::Ole2Frame(_) | EntityType::Underlay(_)
+                )
             })
+            .map(|e| (e.common().handle, e.clone()))
             .collect();
-        for (handle, img) in entries {
-            if let Some(model) = ImageModel::from_raster_image(&img) {
+        for (handle, entity) in entries {
+            if let Some(model) = self.image_seed_for(&entity) {
                 self.images.insert(handle, model);
             }
         }