feat(myopencre): improve CSV import error handling in UI

[PRAteek-singHWY]

⚠️ This PR depends on:

• feat(myopencre): improve CSV import validation (export-compatible) #682 — Backend CSV import validation for MyOpenCRE

Please review this PR after the above PR is merged.

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Scope note (important)

The intended change in this PR is limited to:

• application/frontend/src/pages/MyOpenCRE/MyOpenCRE.tsx

Any other file diffs shown by GitHub are inherited from branch history / stacking and are not part of the frontend error-handling change itself.

Please let me know if you’d prefer this rebased or split further.

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Summary

Closes #584 (partial)

This PR improves the MyOpenCRE frontend experience by surfacing clear, structured feedback for CSV import results, directly reflecting backend validation responses.

The goal is to make CSV imports easier to understand and debug for users, without changing any backend logic.

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What this PR does

Frontend error handling

• Displays backend validation errors directly in the UI
• Handles:
  • Empty CSV uploads
  • Invalid CRE format errors
  • Misaligned CSV columns
  • General validation failures
• Row-level errors are rendered with clear, human-readable messages
  (row number + validation reason)

Success feedback

• Displays a success message after import
• Shows:
  • Number of new CREs added
  • Number of standards imported

UX improvements

• Clears stale success/error messages when a new file is selected
• Prevents ambiguous or misleading upload states
• Keeps frontend behavior tightly aligned with backend responses

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Screenshots

1. Empty CSV upload

2. Invalid CRE entry (bad_cre.csv)

3. Misaligned CSV columns (misaligned_columns.csv)

4. Successful import (valid.csv)

What is intentionally out of scope

• Backend validation logic (handled in feat(myopencre): improve CSV import validation (export-compatible) #682)

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Why this approach

The backend now returns clear, structured validation responses.
This PR ensures those responses are presented cleanly in the UI, making CSV imports:

• Easier to debug
• Less confusing for users
• More production-ready without adding frontend-specific validation logic

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Feedback welcome

If there are UX improvements, edge cases, or error formats that should be handled differently, I’d really appreciate guidance and am happy to iterate.