Adds a "Settings > Auto-update check" submenu with Disabled / Hourly /
Daily / Weekly. Hourly is the default; existing settings files pick it
up automatically via serde default. Manual "Check for updates" is
unchanged and still fires when auto is disabled.
The 24-hour hardcoded interval is replaced by reading
Settings.update_check_interval_secs in both the startup scheduler and
the post-check rearm path. None means auto is disabled and no timer is
armed.
Adds five new i18n keys across all eight locales.
Replaces the "build from source only" Install section with a
Releases-page download path plus a SmartScreen note, and adds
docs/release-process.md so the tag-bump-build flow is captured for
future maintainers.
tiennm99