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/electron/electron_relay.ts

@@ -384,13 +384,33 @@ export async function control_presentation({
 
 // 4. System Management (Phase 5+)
 
-export async function set_wallpaper({ path }: { path: string }) {
+export async function set_wallpaper({
+    path,
+    url,
+    url_external,
+    display = 'all',
+    api_key,
+    account_id
+}: {
+    /** Local file path (existing behavior). */
+    path?: string;
+    /** HTTPS URL — downloaded to ~/Library/Caches/OSIT/wallpaper/ before applying. */
+    url?: string;
+    /** Optional separate URL for the external/projector display only. */
+    url_external?: string;
+    /** Which display(s) to target. Defaults to 'all'. */
+    display?: 'all' | 'primary' | 'external';
+    /** Aether API key passed as x-aether-api-key header on the download request. */
+    api_key?: string;
+    /** Aether account ID passed as x-account-id header on the download request. */
+    account_id?: string;
+}) {
     if (!native || !native.set_wallpaper)
         return {
             success: false,
             error: 'Native handler set_wallpaper not available'
         };
-    return await native.set_wallpaper({ path });
+    return await native.set_wallpaper({ path, url, url_external, display, api_key, account_id });
 }
 
 export async function update_app(args: {