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/lib/stores/ae_events_stores__launcher_defaults.ts

@@ -39,12 +39,17 @@ export interface LauncherLocState {
     section_state__health: SectionState;
     section_state__native_os: SectionState;
     section_state__launch_timing: SectionState;
+    section_state__wallpaper: SectionState;
     section_state__sync_timers: SectionState;
     section_state__updates: SectionState;
     section_state__controller: SectionState;
     section_state__screen_saver: SectionState;
     section_state__app_modes: SectionState;
     section_state__local_actions: SectionState;
+    /** URL last successfully applied as wallpaper on this device. Used to detect remote config changes. */
+    wallpaper_applied_url: string | null;
+    /** URL last applied to the external display (when url_external is configured). */
+    wallpaper_applied_url_external: string | null;
     datetime_format: string;
     time_format: string;
     time_hours: 12 | 24;
@@ -160,6 +165,7 @@ export const launcher_loc_defaults: LauncherLocState = {
     section_state__health: 'auto',
     section_state__native_os: 'collapsed',
     section_state__launch_timing: 'collapsed',
+    section_state__wallpaper: 'collapsed',
     section_state__sync_timers: 'collapsed',
     section_state__updates: 'collapsed',
     section_state__controller: 'auto',
@@ -208,7 +214,9 @@ export const launcher_loc_defaults: LauncherLocState = {
     controller: 'local',
     controller_group_code: 'launcher-00',
     controller_client_id: null,
-    native_test_mode: false
+    native_test_mode: false,
+    wallpaper_applied_url: null,
+    wallpaper_applied_url_external: null
     // controller_cmd: null,
     // controller_trigger_send: null,
 };