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/update/mod.rs

@@ -5087,6 +5087,39 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::FileAssocChanged(enabled) => {
+                // The same two calls FILEASSOC makes, so the checkbox and the
+                // command cannot leave the setting and the registration
+                // disagreeing.
+                self.file_assoc_enabled = enabled;
+                self.persist_settings_if_changed();
+                let outcome = if enabled {
+                    crate::io::file_association::register_as_handler()
+                } else {
+                    crate::io::file_association::unregister_handler()
+                };
+                match outcome {
+                    Ok(()) => {
+                        let said = if enabled {
+                            crate::t!("Registered as a .dwg/.dxf handler.")
+                        } else {
+                            crate::t!("No longer registered as a file handler.")
+                        };
+                        self.command_line.push_output(said.as_ref());
+                    }
+                    Err(why) => {
+                        // The setting is what the user asked for; the
+                        // registration is what the system allowed. Put the
+                        // checkbox back rather than showing a state that is not
+                        // true.
+                        self.file_assoc_enabled = !enabled;
+                        self.persist_settings_if_changed();
+                        self.command_line
+                            .push_error(crate::tf!("File association failed: {why}").as_ref());
+                    }
+                }
+                Task::none()
+            }
             Message::LanguageChanged(language) => {
                 if self.language == language {
                     return Task::none();