feat(app): first-launch prompt to set Open CAD Studio as default for .dwg/.dxf

On the first launch that hasn't answered it yet, the app shows a small
dialog offering to become the default handler for .dwg and .dxf files.
The choice is persisted (default_assoc_prompted) so the prompt never
reappears.

The platform plumbing lives in io/file_association.rs:
  * Windows — opens the OS's own per-app default-programs dialog via
    IApplicationAssociationRegistrationUI::LaunchAdvancedAssociationUI,
    passing the "Open CAD Studio" RegisteredApplications name the MSI
    registers. Modern Windows guards the actual default behind a
    UserChoice hash, so this is the supported consent path.
  * Linux — xdg-mime default against the installed .desktop file, whose
    MimeType entries already cover image/vnd.dwg and image/vnd.dxf.
  * macOS — LSSetDefaultRoleHandlerForContentType binds the DWG/DXF UTIs
    declared in Info.plist to the app bundle id.

The work runs on a dedicated thread (the Windows dialog is modal) with
the result delivered back through a oneshot channel and surfaced in the
command line.

https://claude.ai/code/session_01DFVSKgViFaxEjZD2KsTdtt

src/io/mod.rs

@@ -3,6 +3,7 @@
 // All file reading/writing goes through acadrust.
 // Default save format: DWG (AC1032 / R2018+).
 
+pub mod file_association;
 pub mod obj;
 pub mod pdf_export;
 pub mod plot_style;