feat(launcher): wallpaper auto-apply from device config

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>

src/routes/events/[event_id]/(launcher)/launcher_cfg.svelte

@@ -33,6 +33,7 @@ import Launcher_Cfg_Screen_Saver from './cfg_components/launcher_cfg_screen_save
 import Launcher_Cfg_App_Modes from './cfg_components/launcher_cfg_app_modes.svelte';
 import Launcher_Cfg_Local_Actions from './cfg_components/launcher_cfg_local_actions.svelte';
 import Launcher_Cfg_Launch_Timing from './cfg_components/launcher_cfg_launch_timing.svelte';
+import Launcher_Cfg_Wallpaper from './cfg_components/launcher_cfg_wallpaper.svelte';
 import {
     Bug,
     Code,
@@ -181,6 +182,8 @@ function handle_section_expand(current_key: string) {
                         on_expand={() => handle_section_expand('health')} />
                     <Launcher_Cfg_Native_OS
                         on_expand={() => handle_section_expand('native_os')} />
+                    <Launcher_Cfg_Wallpaper
+                        on_expand={() => handle_section_expand('wallpaper')} />
                     <Launcher_Cfg_Launch_Timing
                         on_expand={() => handle_section_expand('launch_timing')} />
                     {#if $ae_loc.is_native}