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/mod.rs

@@ -1260,6 +1260,8 @@ pub enum Message {
     /// File picker returned. `Some((path, size_in_bytes))` → start loading;
     /// `None` → user cancelled the dialog (no overlay shown).
     OpenPathPicked(Option<(PathBuf, u64)>),
+    /// A file was dragged from the desktop and dropped on the window (#344).
+    FileDropped(PathBuf),
     /// Open a path from the Start tab's recent-documents list (skips the
     /// file picker; the path is already known).
     OpenRecent(PathBuf),