fix(lock): dismiss fade-to-lock overlay when using a custom lock command (#2653)

When a custom lock command is configured, Lock.lock() runs the command and
returns early without engaging WlSessionLock, so IdleService.isShellLocked
never transitions true->false. That transition is the only trigger that
dismisses a completed FadeToLockWindow, so the fully-faded black overlay stays
on screen and the desktop is unusable after re-login (regression from b8f4c35,
which added the _completed guard and tied dismissal solely to isShellLocked).

Add a dedicated dismissFadeToLock signal that the custom-lock branch emits
after launching the external locker, mirroring the existing fade signal wiring,
so the overlay is handed off and torn down. The built-in WlSessionLock path is
unchanged and still dismisses on unlock.

Fixes #2595

quickshell/Modules/Lock/Lock.qml

@@ -60,6 +60,10 @@ Scope {
     function lock() {
         if (SettingsData.customPowerActionLock?.length > 0) {
             Quickshell.execDetached(["sh", "-c", SettingsData.customPowerActionLock]);
+            // The custom locker manages its own surface; DMS never engages
+            // WlSessionLock here, so isShellLocked stays false and the fade
+            // overlay would never be dismissed. Hand off by dismissing it now.
+            IdleService.dismissFadeToLock();
             return;
         }
         if (shouldLock || pendingLock)