Make Vector Asset Studio canvas viewport fill panel height

- update `tools/Vector Asset Studio/svgBackgroundEditor.css`
- set `.canvas-panel` to `min-height: 0` for proper flex/grid shrink behavior
- set `.canvas-viewport` to full available height (`min-height: 0; height: 100%`)
- remove fixed minimum-height behavior that left unused space below the editor
- result: Vector Asset Studio main canvas area now extends to the bottom of its panel consistently

Author: DavidQ

tools/Vector Asset Studio/svgBackgroundEditor.css

@@ -244,6 +244,7 @@ button:disabled {
   display: flex;
   flex-direction: column;
   padding: 10px;
+  min-height: 0;
 }
 
 .canvas-toolbar {
@@ -258,7 +259,8 @@ button:disabled {
 .canvas-viewport {
   position: relative;
   flex: 1;
-  min-height: 260px;
+  min-height: 0;
+  height: 100%;
   border: 1px solid var(--svg-border);
   border-radius: 8px;
   overflow: hidden;