refactor(context): simplify refresh handling

[alikhere]

Summary

The fetchData function in DataContext was doing nothing useful — it just flipped refresh back to false inside a try/catch that could never actually throw. This created a pointless useEffect → useCallback → state update cycle on every refresh toggle without fetching any data.

Removed fetchData and the useEffect that called it. The context now just exposes refresh and setRefresh as plain state, and individual components own their own fetch behavior when the flag changes. Also dropped the unused useCallback and useEffect imports since nothing in the file needs them anymore.

How to test

• cd webapp && npm run test — all Vitest suites pass
• cd webapp && npm run build — Vite builds successfully
• cd gdbui_server && python -m unittest discover -s . -p 'flask_test.py'
  — all backend tests pass
• Open the app and confirm panels that use refresh still update
  correctly when debug commands are triggered

Related Issue

Fixes #163

Notes

[alikhere]

Hi @Shubh942 Could you please review this PR.