feat(app): silently register as a .dwg/.dxf handler so portable builds show in "Open with"

Installed builds already register a handler (MSI ProgIDs, the Linux
.desktop MimeType, the macOS Info.plist), but the portable .exe and the
AppImage register nothing, so they never appear in the OS "Open with"
list. Add a best-effort, idempotent runtime registration that runs
silently on a detached thread at startup:

  * Windows — writes HKCU\Software\Classes\Applications\OpenCADStudio.exe
    (open command + FriendlyAppName + SupportedTypes .dwg/.dxf), which is
    what lists the exe in "Open with → Choose another app".
  * Linux — writes a user-level .desktop pointing at the running binary
    ($APPIMAGE when set) and refreshes the desktop database. Skipped when
    a system package already provides the desktop entry, so we never
    shadow a distro install.
  * macOS — no-op; .app bundles auto-register with LaunchServices.

Unlike the default-app prompt this changes no defaults and shows no UI;
it only makes the binary discoverable as a handler.

https://claude.ai/code/session_01DFVSKgViFaxEjZD2KsTdtt

src/app/mod.rs

@@ -1569,6 +1569,15 @@ impl OpenCADStudio {
     /// opens the primary application window.
     fn boot() -> (Self, Task<Message>) {
         use helpers::build_window_icon;
+        // Silently (re)register this binary as a .dwg/.dxf handler so it appears
+        // in the OS "Open with" list. Best-effort and idempotent; covers the
+        // portable .exe / AppImage that no installer has registered. Detached so
+        // it never delays startup.
+        std::thread::spawn(|| {
+            if let Err(e) = crate::io::file_association::register_as_handler() {
+                eprintln!("file association: handler registration failed: {e}");
+            }
+        });
         let state = Self::new();
         let (id, open_task) = window::open(window::Settings {
             maximized: true,