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.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/notifications/mod.rs

@@ -70,7 +70,7 @@ impl NotifServer {
         (
             "breadbar".into(),
             "breadway".into(),
-            "0.1.0".into(),
+            env!("CARGO_PKG_VERSION").into(),
             "1.2".into(),
         )
     }