fix(services/spotlight): improve MPRIS artwork handling and scope keyword searches to single plugin

- mpris: native MediaArt cache lookup, dedupe play/pause emission, safe
  cleanup, artwork polling for already-playing browser players, drop
  os.system seek fallback
- spotlight: scope plugin keyword and detect() matches to a single plugin
  via search_single instead of global search; register clipboard keywords
  (clip/clipboard)

src/window/lock.py

@@ -257,12 +257,12 @@ def initialize():
         ),
     )
     lockscreen.show()
+    return lock, lockscreen
 
 
 def main():
     set_process_name("lockscreen")
-    initialize()
-    lockscreen = LockScreen(GtkSessionLock.Lock())
+    _lock, lockscreen = initialize()
 
     global app
     app = Application("lock", lockscreen)