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.

src/lockscreen.rs

@@ -27,9 +27,6 @@ pub struct LockscreenHandles {
     pub password_entry: gtk::PasswordEntry,
     pub unlock_callback: Rc<dyn Fn()>,
     pub username: String,
-    /// The monitor this window was assigned to (session-lock path only).
-    /// Used to prune the window when its monitor is removed on suspend/resume.
-    pub monitor: Option<gdk::Monitor>,
     state: Rc<RefCell<LockscreenState>>,
 }
 
@@ -73,7 +70,6 @@ pub fn create_lockscreen_window(
                 password_entry: gtk::PasswordEntry::new(),
                 unlock_callback,
                 username: String::new(),
-                monitor: None,
                 state: Rc::new(RefCell::new(LockscreenState {
                     failed_attempts: 0,
                     fp_listener_rc: None,
@@ -364,7 +360,6 @@ pub fn create_lockscreen_window(
         password_entry: password_entry.clone(),
         unlock_callback,
         username: user.username,
-        monitor: None,
         state: state.clone(),
     }
 }