Fix inconsistent spacing and label display in reading heatmap chart

[acx10]

🚀 Pull Request

📝 Description

🛠️ Changes Implemented

🧪 Testing Strategy

📸 Visual Changes (if applicable)

⚠️ Required Pre-Submission Checklist

• Code adheres to project style guidelines and conventions
• Branch synchronized with latest develop branch
• Automated unit/integration tests added/updated to cover changes
• All tests pass locally (./gradlew test for backend)
• Manual testing completed in local development environment
• Flyway migration versioning follows correct sequence (if database schema modified)
• Documentation pull request submitted to booklore-docs (required for features or enhancements that introduce user-facing or visual changes)

💬 Additional Context (optional)