Add file type association manager with MIME type support

Generated .desktop files now include MimeType and StartupWMClass when
available. Detail view system tab shows MIME types with per-type
"Set Default" buttons that use xdg-mime and track previous defaults
in system_modifications for reversal.

src/ui/detail_view.rs

@@ -1085,6 +1085,60 @@ fn build_system_tab(record: &AppImageRecord, db: &Rc<Database>, toast_overlay: &
         }
     }
 
+    // File type associations group
+    if let Some(ref mime_str) = record.mime_types {
+        let types: Vec<&str> = mime_str.split(';').filter(|s| !s.is_empty()).collect();
+        if !types.is_empty() {
+            let mime_group = adw::PreferencesGroup::builder()
+                .title("File Type Associations")
+                .description("MIME types this app can handle. Set as default to open these files with this app.")
+                .build();
+
+            let app_id = integrator::make_app_id(
+                record.app_name.as_deref().unwrap_or(&record.filename),
+            );
+
+            for mime_type in &types {
+                let row = adw::ActionRow::builder()
+                    .title(*mime_type)
+                    .build();
+
+                let set_btn = gtk::Button::builder()
+                    .label("Set Default")
+                    .valign(gtk::Align::Center)
+                    .build();
+                set_btn.add_css_class("flat");
+
+                let db_mime = db.clone();
+                let record_id = record.id;
+                let app_id_clone = app_id.clone();
+                let mime = mime_type.to_string();
+                let toast_mime = toast_overlay.clone();
+                set_btn.connect_clicked(move |btn| {
+                    match integrator::set_mime_default(
+                        &db_mime, record_id, &app_id_clone, &mime,
+                    ) {
+                        Ok(()) => {
+                            toast_mime.add_toast(adw::Toast::new(
+                                &format!("Set as default for {}", mime),
+                            ));
+                            btn.set_sensitive(false);
+                            btn.set_label("Default");
+                        }
+                        Err(e) => {
+                            log::error!("Failed to set MIME default: {}", e);
+                            toast_mime.add_toast(adw::Toast::new("Failed to set default"));
+                        }
+                    }
+                });
+
+                row.add_suffix(&set_btn);
+                mime_group.add(&row);
+            }
+            inner.append(&mime_group);
+        }
+    }
+
     // Runtime Compatibility group
     let compat_group = adw::PreferencesGroup::builder()
         .title("Compatibility")