feat(launcher): restore macOS default wallpaper + external-only apply fix

- electron_relay: add restore_macos_default_wallpaper() — uses run_osascript
  to find first .heic in /System/Library/Desktop Pictures/ (version-agnostic)
- wallpaper cfg: Restore macOS Default button (native or edit_mode); clears
  applied-URL tracking so next configured URL re-applies correctly
- wallpaper cfg: fix Apply Now / Save & Apply enabled when only external URL
  is filled; display target becomes 'external' to leave built-in unchanged

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/electron/electron_relay.ts

@@ -413,6 +413,38 @@ export async function set_wallpaper({
     return await native.set_wallpaper({ path, url, url_external, display, api_key, account_id });
 }
 
+/**
+ * Restores the macOS default wallpaper on all displays.
+ * Scans /System/Library/Desktop Pictures/ for the first .heic file — works across
+ * all recent macOS versions without needing to know the version name.
+ * No-op on non-macOS (Linux/Windows return success:false from run_osascript).
+ */
+export async function restore_macos_default_wallpaper(
+    display: 'all' | 'primary' | 'external' = 'all'
+): Promise<{ success: boolean; error?: string }> {
+    const display_target =
+        display === 'primary'
+            ? 'tell desktop 1'
+            : display === 'external'
+              ? 'tell desktop 2'
+              : 'tell every desktop';
+
+    const script = `
+set pic_path to do shell script "ls '/System/Library/Desktop Pictures/'*.heic 2>/dev/null | head -1"
+if pic_path is "" then
+    error "No default macOS wallpaper (.heic) found in /System/Library/Desktop Pictures/"
+end if
+tell application "System Events"
+    ${display_target}
+        set picture to pic_path
+    end tell
+end tell
+`.trim();
+
+    const result = await run_osascript(script);
+    return result ?? { success: false, error: 'Native bridge not available' };
+}
+
 export async function update_app(args: {
     source: 'url' | 'file';
     url?: string;