feat(gui): forward --relay flag to host/viewer children

gui::run() dropped the parsed --relay value, so a relay chosen on the
GUI command line (pixelpass --gui --relay URL) never reached the
headless host/viewer children -- only the PIXELPASS_RELAY env-var form
propagated (via inheritance). Thread the flag into the app and append
--relay <url> to both child arg vectors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main.rs

@@ -25,7 +25,7 @@ async fn main() -> Result<()> {
     if cli.gui {
         #[cfg(feature = "gui")]
         {
-            return gui::run();
+            return gui::run(cli.relay);
         }
         #[cfg(not(feature = "gui"))]
         {