fix: quit in ::unlocked instead of after unlock() (v0.6.17)

The real cause of the unlock SIGSEGV: gtk4-session-lock destroys the lock
windows itself when the lock ends (per its header), so unlock_callback's
`unlock(); app.quit();` destroyed them a second time — surface already gone
→ gdk_surface_get_display NULL → crash in gtk_window_destroy. Reproduces on
a plain single-monitor lock/unlock, no suspend needed.

unlock_callback now calls only unlock(); a new connect_unlocked handler
quits the app after the library finishes teardown. Mirrors the upstream
examples/simple.rs exactly.

Reverts the v0.6.15/v0.6.16 monitor-pruning + LockscreenHandles.monitor:
it was a misdiagnosis (the crash is monitor-independent) and manually
manipulated library-managed lock windows, which the upstream example
explicitly warns against. Monitor removal is left to the library.

Cargo.lock

@@ -575,7 +575,7 @@ dependencies = [
 
 [[package]]
 name = "moonlock"
-version = "0.6.16"
+version = "0.6.17"
 dependencies = [
  "gdk-pixbuf",
  "gdk4",