feat: add system tray (StatusNotifierWatcher / SNI)

Implements org.kde.StatusNotifierWatcher as a D-Bus service so apps
like Nextcloud can register their tray icons. Icons are rendered from
SNI ARGB pixmaps (falling back to icon-name theme lookup), click calls
Activate(0,0), and NameOwnerChanged cleans up ghost icons when an app
exits. Styling follows the Bread Design System (4px tertiary radius,
xs/sm spacing, opacity transitions).

Also fixes a latent infinite-loop risk in osd.rs (.flatten → .map_while)
and syncs the notifications server version string to CARGO_PKG_VERSION.

src/osd.rs

@@ -35,7 +35,7 @@ fn volume_watcher(tx: mpsc::Sender<OsdEvent>) {
     let stdout = child.stdout.take().unwrap();
     let reader = BufReader::new(stdout);
 
-    for line in reader.lines().flatten() {
+    for line in reader.lines().map_while(Result::ok) {
         if line.contains("'change' on sink") {
             if let Some(evt) = query_volume() {
                 let _ = tx.blocking_send(evt);