feat: Enhance brush settings and add new layer paste functionality

- Added brush color variant and variant amount settings to the brush panel.
- Introduced a checkbox for color variant and a slider for variant amount in the brush settings.
- Implemented "Paste as New Layer" functionality, allowing users to paste clipboard images directly onto a new layer.
- Updated menus to include the new paste option with a shortcut.
- Improved layer panel iconography by replacing text buttons with SVG icons for visibility and lock toggles.
- Created new SVG assets for closed eye and watercolor icons.
- Enhanced history panel to provide richer descriptions for brush strokes and vector shape modifications.
- Fixed the issue where the first brush stroke was missing from the history panel.

hcie-engine-api/src/stroke_cache.rs

@@ -62,16 +62,28 @@ pub struct PendingHistoryItem {
 }
 
 impl Engine {
+    /// Returns true if at least one background-thread history snapshot is still
+    /// waiting to be committed to the document history.
+    ///
+    /// This is used by the GUI to decide whether to schedule an additional
+    /// delayed `commit_pending_history` poll after a brush stroke ends.
+    pub fn has_pending_history(&self) -> bool {
+        !self.pending_history.lock().unwrap().is_empty()
+    }
+
     /// Commit any pending history items computed in the background thread.
     /// This should be polled on the UI thread.
     ///
     /// Also recycles returned buffers back into the engine's pool to avoid
     /// allocations on subsequent strokes.
-    pub fn commit_pending_history(&mut self) {
+    ///
+    /// Returns `true` if at least one pending snapshot was committed on this
+    /// call, `false` if the queue was empty.
+    pub fn commit_pending_history(&mut self) -> bool {
         let items = {
             let mut pending = self.pending_history.lock().unwrap();
             if pending.is_empty() {
-                return;
+                return false;
             }
             std::mem::take(&mut *pending)
         };
@@ -106,6 +118,7 @@ impl Engine {
                 );
             }
         }
+        true
     }
 
     /// Begin a new brush stroke on the given layer.
@@ -259,6 +272,7 @@ impl Engine {
                         None
                     };
 
+                    let style = self.current_tip.style;
                     let bounds = self.last_stroke_bounds;
                     let pending_history = self.pending_history.clone();
 
@@ -275,7 +289,10 @@ impl Engine {
                             bounds: None,
                             before_shapes,
                             after_shapes,
-                            description: "Brush Stroke".to_string(),
+                            description: format!(
+                                "Brush Stroke ({})",
+                                crate::stroke_brush::brush_style_label(style)
+                            ),
                             return_before: None,
                             return_after: None,
                         };