feat(open): drag & drop files onto the window (#344)

Dropping a supported file (dwg / dxf / bak / sv$, case-insensitive)
opens it through the existing open flow: an already-open drawing
switches to its tab, a load in progress queues the drop behind it
(multi-file drops arrive one event each), anything unsupported reports
on the command line instead of failing silently in the parser.

Platform support follows winit 0.30: Windows, macOS and Linux/X11
(incl. XWayland) deliver FileDropped; the Wayland backend does not
implement file drag & drop yet, so drops are inert there until winit
gains it.

Closes #344

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/view/mod.rs

@@ -1732,6 +1732,9 @@ impl OpenCADStudio {
                     iced::Event::Window(window::Event::Resized(sz)) => {
                         Some(Message::WindowResized(sz.width as f32, sz.height as f32))
                     }
+                    iced::Event::Window(window::Event::FileDropped(path)) => {
+                        Some(Message::FileDropped(path))
+                    }
                     iced::Event::Keyboard(keyboard::Event::ModifiersChanged(m)) => {
                         // `command()` is Cmd on macOS, Ctrl elsewhere — the
                         // platform multi-select modifier for the layer list.