Stores wallpaper URL(s) in event_device.other_json.launcher.wallpaper and
auto-applies on all native Launcher instances within one heartbeat cycle (~60s),
eliminating manual per-Mac setup at events.
- electron_relay: typed set_wallpaper wrapper (url, url_external, display, auth headers)
- launcher_defaults: wallpaper_applied_url tracking + section_state__wallpaper
- launcher_cfg_wallpaper: new config section — save to device config + apply now
- launcher_cfg: add wallpaper section to device tab
- launcher_background_sync: auto-apply if config URL changed since last apply;
external-only config targets only the secondary display, leaving built-in unchanged
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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