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/capabilities/migrated.json

@@ -48,6 +48,9 @@
     "notification:default",
     "core:app:allow-app-show",
     "core:app:allow-app-hide",
+    "autostart:allow-enable",
+    "autostart:allow-disable",
+    "autostart:allow-is-enabled",
     {
       "identifier": "opener:allow-open-url",
       "allow": [{ "url": "http://*" }, { "url": "https://*" }]