Add system tray / minimize to tray (roadmap #8)

- tauri "tray-icon" feature enabled
- Tray icon created on startup using the app's default window icon
- Tray menu: "Open Wrystr" (show + focus) and "Quit" (app.exit)
- Left-click on tray icon: show + focus the main window
- Right-click: context menu (platform default)
- Window close button (X) now hides to tray instead of exiting —
  "Quit" in the tray menu is the exit point
- Works on Windows (system tray), Linux (status bar tray), macOS
  (menu bar); DE-specific tray support varies on Linux

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

src-tauri/Cargo.toml

@@ -18,7 +18,7 @@ crate-type = ["staticlib", "cdylib", "rlib"]
 tauri-build = { version = "2", features = [] }
 
 [dependencies]
-tauri = { version = "2", features = ["devtools"] }
+tauri = { version = "2", features = ["devtools", "tray-icon"] }
 tauri-plugin-opener = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"