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/Services/IdleService.qml

@@ -53,6 +53,7 @@ Singleton {
     signal lockRequested
     signal fadeToLockRequested
     signal cancelFadeToLock
+    signal dismissFadeToLock
     signal fadeToDpmsRequested
     signal cancelFadeToDpms
     signal requestMonitorOff