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/core/integrator.rs

@@ -104,7 +104,10 @@ pub fn integrate(record: &AppImageRecord) -> Result<IntegrationResult, Integrati
 
     let icon_id = format!("driftwood-{}", app_id);
 
-    let desktop_content = format!("\
+    let mime_types = record.mime_types.as_deref().unwrap_or("");
+    let wm_class = record.startup_wm_class.as_deref().unwrap_or("");
+
+    let mut desktop_content = format!("\
 [Desktop Entry]
 Type=Application
 Name={name}
@@ -128,6 +131,13 @@ X-AppImage-Integrated-Date={date}
         date = now,
     );
 
+    if !mime_types.is_empty() {
+        desktop_content.push_str(&format!("MimeType={}\n", mime_types));
+    }
+    if !wm_class.is_empty() {
+        desktop_content.push_str(&format!("StartupWMClass={}\n", wm_class));
+    }
+
     fs::write(&desktop_path, &desktop_content)?;
 
     // Install icon if we have a cached one
@@ -340,6 +350,51 @@ pub fn undo_all_modifications(db: &Database, appimage_id: i64) -> Result<(), Str
     Ok(())
 }
 
+/// Set an AppImage as the default handler for a MIME type.
+/// Stores the previous default in system_modifications for reversal.
+pub fn set_mime_default(
+    db: &Database,
+    appimage_id: i64,
+    app_id: &str,
+    mime_type: &str,
+) -> Result<(), String> {
+    let desktop_filename = format!("driftwood-{}.desktop", app_id);
+
+    // Query current default
+    let prev = Command::new("xdg-mime")
+        .args(["query", "default", mime_type])
+        .output()
+        .ok()
+        .and_then(|o| {
+            if o.status.success() {
+                let s = String::from_utf8_lossy(&o.stdout).trim().to_string();
+                if s.is_empty() { None } else { Some(s) }
+            } else {
+                None
+            }
+        });
+
+    // Set new default
+    let status = Command::new("xdg-mime")
+        .args(["default", &desktop_filename, mime_type])
+        .status()
+        .map_err(|e| format!("xdg-mime failed: {}", e))?;
+
+    if !status.success() {
+        return Err("xdg-mime returned non-zero".to_string());
+    }
+
+    // Track the modification
+    db.register_modification(
+        appimage_id,
+        "mime_default",
+        mime_type,
+        prev.as_deref(),
+    ).ok();
+
+    Ok(())
+}
+
 fn update_desktop_database() {
     let apps_dir = applications_dir();
     Command::new("update-desktop-database")

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")