Add drag-and-drop for presets and colors, max_colors validation, and 2D grid layout

- Add drag-and-drop to reorder presets in tabs (2D grid layout)
- Add drag-and-drop to reorder colors within presets
- Add max_colors field to pattern definitions
- Hide color section when max_colors is 0
- Validate color count against pattern max_colors limit
- Store presets in 2D grid format (3 columns)
- Remove left panel from tab content, show only presets
- Update color palette to show swatches instead of hex codes
- Improve preset editor UI with visual color swatches

src/static/style.css

@@ -364,7 +364,7 @@ header h1 {
 
 .presets-list {
     display: grid;
-    grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
+    grid-template-columns: repeat(3, 1fr);
     gap: 0.75rem;
 }