feat: Add theme and font management for egui GUI

- Introduced `theme.rs` to handle visual themes, font discovery, and application of theme colors.
- Implemented system font registration for Linux, macOS, and Windows.
- Added `apply_theme` function to configure egui visuals based on selected theme presets.

feat: Implement interactive tool state management

- Created `tool_state.rs` to manage runtime tool, selection, and transform states.
- Defined `ToolState` struct to encapsulate active tool, color settings, drawing states, and AI panel states.
- Included functionality for managing brush presets, text editing, and selection transformations.

feat: Add utility functions for image and file handling

- Developed `utils.rs` with stateless helper functions for color formatting, drawing icons, and cropping images.
- Implemented save dialog builders and error handling for file operations.
- Added flood fill functionality for composite RGBA pixels.

test: Add unit test for bitmap brush stamp functionality

- Created `bitmap_brush_stamp.rs` to verify the behavior of bitmap brush stamping in the engine.
- Ensured that the painted area matches expected dimensions and characteristics.

hcie-draw/src/lib.rs

@@ -64,6 +64,19 @@ pub fn draw_brush_stroke(
         None
     };
 
+    // Pre-generate a bitmap stamp once for the whole stroke. Bitmap brushes
+    // ignore the procedural shape and use the imported/sampled alpha mask.
+    let bitmap_stamp = if tip.style == BrushStyle::Bitmap
+        && !tip.bitmap_pixels.is_empty()
+        && tip.bitmap_w > 0
+        && tip.bitmap_h > 0
+    {
+        Some(hcie_brush_engine::generate_brush_stamp(tip))
+    } else {
+        None
+    };
+    let stamp_ref = bitmap_stamp.as_deref();
+
     let mut last_p = points[0];
     let (jx0, jy0) = jitter_offset(tip.jitter_amount, tip.size);
     let first_x = last_p.0 + jx0;
@@ -79,10 +92,11 @@ pub fn draw_brush_stroke(
         first_color[0], first_color[1], first_color[2],
         (first_color[3] as f32 * first_opacity).round() as u8,
     ];
-    hcie_brush_engine::draw_dab_capped(
+    hcie_brush_engine::draw_dab_capped_with_stamp(
         &mut layer.pixels, layer.width, layer.height,
         first_x, first_y, first_size, tip.hardness, first_px_color, 1.0, is_eraser, mask,
         stroke_mask.as_deref_mut(), bg_pixels, tip.opacity,
+        stamp_ref,
     );
 
     for &p in &points[1..] {
@@ -116,10 +130,11 @@ pub fn draw_brush_stroke(
                 dab_color[0], dab_color[1], dab_color[2],
                 (dab_color[3] as f32 * pressure_opacity).round() as u8,
             ];
-            hcie_brush_engine::draw_dab_capped(
+            hcie_brush_engine::draw_dab_capped_with_stamp(
                 &mut layer.pixels, layer.width, layer.height,
                 final_x, final_y, effective_size, tip.hardness, px_color, 1.0, is_eraser, mask,
                 stroke_mask.as_deref_mut(), bg_pixels, tip.opacity,
+                stamp_ref,
             );
         }