Dim and sleep locked displays after inactivity

Add adjustable lock-only inactivity delays, a smooth dimming fade and DRM display sleep without changing output layout. Input, session resume and policy changes wake displays. Pending flips retire before power-off, and protected fallback frames preserve the policy after an early locker crash while unlock ownership checks remain strict.

Reduce the password field and avatar, expose delays in Settings, document the practical swaylock comparison and remove the completed backlog.

Validation: independent review cleared; 255 compositor tests, 7 locker tests and 15 Settings tests pass; affected debug binaries rebuilt. Isolated visual smoke remains blocked by an unlocked nested screenshot timeout, matching the previously established baseline presentation limitation. Real DRM display sleep requires a live session check.

compositor/src/winit.rs

@@ -1148,6 +1148,30 @@ fn output_elements(
             });
         background.resize(geometry.size);
         let mut elements = Vec::new();
+        let opacity = state.session_lock.idle_opacity;
+        if opacity > 0.0 {
+            let overlay = state
+                .session_lock
+                .idle_overlays
+                .entry(output.clone())
+                .or_insert_with(|| {
+                    smithay::backend::renderer::element::solid::SolidColorBuffer::new(
+                        geometry.size,
+                        [0.0, 0.0, 0.0, 1.0],
+                    )
+                });
+            overlay.resize(geometry.size);
+            elements.push(
+                SolidColorRenderElement::from_buffer(
+                    overlay,
+                    (0, 0),
+                    scale,
+                    opacity,
+                    RenderElementKind::Unspecified,
+                )
+                .into(),
+            );
+        }
         if let Some(surface) = state
             .session_lock
             .surfaces