feat(desktop): add amphetamine-style keep-awake for mac/computer

- Add `KeepAwakeController` module to hold OS power assertion (idle + system sleep) via the `keepawake` crate, blocking idle sleep while display sleep remains allowed
- Add keep-awake checkbox to Settings control window with platform-specific label ("Keep Mac awake" on macOS, "Keep computer awake" elsewhere)
- Add keep-awake toggle to tray menu as a `CheckMenuItem` for quick access
- Persist keep-awake preference in `desktop-prefs.json` and restore on launch
- Show lightning badge overlay on tray icon when keep-awake is enabled, with corresponding tooltip ("Stitch — keeping awake")
- Update control UI HTML and state-sync script to include keep-awake state
- Release keep-awake assertion on app quit
- Add `keepawake` 0.6 as optional dependency (gated behind `desktop` feature)
- Update installation docs to explain the feature and its persistence behavior
- Add unit tests for badge rendering and controller state management

Author: bnolens

.textile-monorepo-source

@@ -1 +1 @@
-6819314f4b44a974b3920dd880f20448bc13ca97
+412b30ea7b1b91ee738f97729df48925cc38bf60

.textile-stitch-release-version

@@ -1 +1 @@
-0.1.147
+0.1.148