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")