fix(file_handlers): use keystroke Cmd+Return to start pptx slideshow

Replace unreliable AppleScript PowerPoint API (run slide show of settings)
with System Events keystroke approach, matching proven behavior from the
old MasterKey app. Opens the file, waits 3s for load, then sends Cmd+Return
to start the slideshow from slide 1.

dist/main/file_handlers.js

@@ -174,8 +174,10 @@ function registerFileHandlers() {
               tell application "Microsoft PowerPoint"
                 activate
                 open (POSIX file "${target}")
-                delay 1
-                run slide show of active presentation
+                delay 3
+              end tell
+              tell application "System Events"
+                keystroke return using command down
               end tell
             `;
                     }