Add a "keep computer awake" toggle (Claude-style) for long/overnight runs:
the renderer owns the device-local pref and mirrors it to the Electron main
process, which holds a single `powerSaveBlocker('prevent-app-suspension')` —
the same authority split as translucency. Surfaced as a statusbar quick-toggle
and a Settings row.
Introduce a dedicated System settings section (device-local machine prefs) and
de-crowd Appearance by moving Window Translucency + UI Scale into it (both are
main-process/window-owned, not visual theme). Give Haptic Feedback its first
Settings home there too (the titlebar quick-toggle stays). Relocated i18n copy
into `settings.system` across en/zh/zh-hant/ja; wired the `system` route into
the SettingsView union + allowlist + nav.
