fix: Resolve TypeScript type narrowing errors in drag-to-paint tests

TypeScript was narrowing the `paintMode` variable type from
`'on' | 'off' | null` to just `'off'`, causing TS2367 errors when
comparing to 'on'. Fixed by casting the value to the full union type.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: adewale

app/src/components/drag-to-paint.test.tsx

@@ -85,7 +85,8 @@ describe('Paint Mode State Management', () => {
 
     it('should toggle step to inactive when paintMode is "off" and step is active', () => {
       const steps = [true, true, true, true];
-      const paintMode: 'on' | 'off' | null = 'off';
+      // Cast to union type to prevent TypeScript from narrowing to literal
+      const paintMode = 'off' as 'on' | 'off' | null;
       const toggledSteps: number[] = [];
 
       const handlePaintEnter = (stepIndex: number) => {
@@ -125,7 +126,8 @@ describe('Paint Mode State Management', () => {
 
     it('should NOT toggle step when paintMode is "off" and step is already inactive', () => {
       const steps = [false, false, false, false];
-      const paintMode: 'on' | 'off' | null = 'off';
+      // Cast to union type to prevent TypeScript from narrowing to literal
+      const paintMode = 'off' as 'on' | 'off' | null;
       const toggledSteps: number[] = [];
 
       const handlePaintEnter = (stepIndex: number) => {