feat(native): Linux parity + autostart + tray DND (P6-1; no macOS)

Rounds out the native app on Linux (Windows features kept; macOS stays no-op):
- power.rs: no-sleep during calls on Linux via a zbus org.freedesktop.ScreenSaver
  Inhibit/UnInhibit (cookie held in ScreenSaverInhibit managed state).
- set_badge_count: Linux launcher badge via the Unity
  com.canonical.Unity.LauncherEntry.Update D-Bus signal (best-effort; app_uri
  = cinny.desktop per Tauri's mainBinaryName naming).
- tauri-plugin-autostart registered (+ autostart:allow-enable/disable/is-enabled
  capabilities); web toggles it from Settings.
- Tray "Do Not Disturb" CheckMenuItem → emits lotus-dnd-changed to the web,
  which ORs it into the notification quiet-gate.

zbus 5 (Linux target dep; blocking-api default). CI-compile-verified
(windows+linux); reviewer confirmed no build-breakers. Runtime to check on
Linux: DND toggle polarity, badge .desktop id.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -35,6 +35,14 @@ custom-protocol = [ "tauri/custom-protocol" ]
 [target.'cfg(not(any(target_os = "android", target_os = "ios")))'.dependencies]
 tauri-plugin-updater = "2"
 tauri-plugin-single-instance = "2"
+tauri-plugin-autostart = "2" # P6-1 launch-on-login
+
+[target.'cfg(target_os = "linux")'.dependencies]
+# P6-1 desktop parity: screensaver inhibit (no-sleep in calls) + Unity launcher
+# badge, both via the session D-Bus. zbus 5.x ships the blocking API under the
+# default `blocking-api` feature and the default `async-io` runtime, so plain
+# default features suffice (no tokio integration needed here).
+zbus = "5"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 webview2-com = "0.38"