feat(options): offer the file association in Options

Registering as the .dwg/.dxf handler was reachable only by typing
FILEASSOC, which is also the only thing that installs the icons the
desktop shows — so a user with no reason to know the command had no way
to arrive at either.

The checkbox makes the same two calls the command does, so the stored
setting and the actual registration cannot drift apart. A registration
the system refuses puts the checkbox back rather than leaving it showing
a state that is not true.

The line under it says the icons come with it, since that is what sends
people looking for this in the first place.

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

src/app/mod.rs

@@ -1767,6 +1767,9 @@ pub enum Message {
     OptionsThemeChanged(String),
     /// Edit one of Custom theme's six base colours as #RRGGBB.
     OptionsThemeColorChanged(usize, String),
+    /// Register or unregister as the .dwg/.dxf handler, from Options. Same
+    /// setting the FILEASSOC command carries.
+    FileAssocChanged(bool),
     /// Switch the interface language and redraw localized views.
     LanguageChanged(crate::i18n::Language),
     /// Drop every entity from the active drawing.