Integrate category CRUD APIs

[gTanusri]

Summary

• Integrated the existing category UI with backend Category APIs.
• Added active category listing with loading, empty/error handling, and refresh support.
• Added edit and delete actions for categories in the sidebar list.
• Reused the category modal for both create and update flows.
• Added frontend validation, delete confirmation, and success/error toast feedback.

Issue

Closes #230

Validation

• npm run build passes.
• ./gradlew.bat compileJava passes.
• git diff --check passes.