fix(desktop): enable tauri tray-icon feature

tauri::tray is gated behind the 'tray-icon' Cargo feature; the tray code
failed to compile without it (E0432). Also enable image-png for tray icon
decoding.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -17,7 +17,7 @@ tauri-build = { version = "2", features = [] }
 [dependencies]
 serde_json = "1.0.109"
 serde = { version = "1.0.193", features = ["derive"] }
-tauri = { version = "2", features = ["devtools", "wry"] }
+tauri = { version = "2", features = ["devtools", "wry", "tray-icon", "image-png"] }
 tauri-plugin-localhost = "2"
 tauri-plugin-window-state = "2"
 tauri-plugin-clipboard-manager = "2"