fix(practice): prevent stray "0" rendering in problem area

Fix JSX gotcha where `problemHeight && <Component />` would render "0"
when problemHeight is 0. Changed to `(problemHeight ?? 0) > 0 &&` which
always evaluates to a boolean.

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

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

apps/web/src/components/practice/ActiveSession.tsx

@@ -1480,7 +1480,7 @@ export function ActiveSession({
               )}
               {/* Abacus dock - positioned absolutely so it doesn't affect problem centering */}
               {/* Width 100% matches problem width, height matches problem height */}
-              {currentPart.type === 'abacus' && !showHelpOverlay && problemHeight && (
+              {currentPart.type === 'abacus' && !showHelpOverlay && (problemHeight ?? 0) > 0 && (
                 <AbacusDock
                   id="practice-abacus"
                   columns={String(Math.abs(attempt.problem.answer)).length}