fix: use defaultValue for interactive abacus control

Changed from controlled (value) to uncontrolled (defaultValue) pattern
to allow AbacusReact to fully manage its own state and be interactive.
Still tracks value changes via onValueChange callback for display.

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

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

apps/web/src/app/page.tsx

@@ -76,7 +76,7 @@ export default function HomePage() {
                   Click the beads to interact!
                 </div>
                 <AbacusReact
-                  value={abacusValue}
+                  defaultValue={1234567}
                   columns={7}
                   beadShape="diamond"
                   colorScheme="place-value"