1. ThemeSettings: change 'auto' radio value to 'system' to match
the v5 settingsSchema enum. Storing 'auto' silently wipes all
settings on next launch.
2. UpdateBanner: render an 'available' branch so users see a CTA
when a new version is detected (the store already had this state,
but the banner was silent). Added a test.
3. feed-config: remove the unused resolveFeedUrl / FEEDS exports.
feedConfigFor is the actual implementation used by the IPC
handler. Updated tests to match.
4. main/index.js: tighten app:open-external regex to https:// only.
amitwh