fixed: Replaced Scans alert() failures with toast feedback

[pranavv2707]

Description

Replaced alert() dialogs in the Scans page destructive-action failure paths with accessible toast notifications using the existing ToastContext. Also adds test coverage for all three failure paths:

• Delete single task — clicking "Delete_Record" on one scan and confirming
• Bulk delete — selecting multiple scans and clicking "Prune_Selected_Records"
• Clear all — clicking "Purge_All_Records" which wipes everything

Related Issues

Closes issue number #469

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

Unit tests — 5 new tests in Scans.failures.test.tsx:

• deleteTask failure shows error toast and keeps task in list
• bulkDeleteTasks failure shows error toast and keeps tasks in list
• clearAllTasks failure shows error toast

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

Issues Fixed

1. The Scans page was using browser alert() dialogs for delete and clear-history failure paths, which block the page, cannot be styled, and are inaccessible to screen readers.
   • Replaced all alert() calls in handleTaskDelete, handleClearAll, and handleBulkDelete failure paths with addToast() from the existing ToastContext.
   • Error toasts use role="alert" so screen readers announce them automatically.
   • Console logging kept for diagnostics.

Tests Added

• frontend/testing/unit/pages/Scans.failures.test.tsx — 5 new tests covering:
  • deleteTask failure shows error toast and keeps task in list
  • bulkDeleteTasks failure shows error toast and keeps tasks in list
  • clearAllTasks failure shows error toast

How to Test

1. Run cd frontend && npm run test — all 5 new tests should pass.
2. Run npm run build — build should complete with no errors.
3. Optionally, run the frontend locally, open the Scans page, and trigger a delete while the backend is stopped — an error toast should appear instead of a browser alert popup.

Comments:

The backend-lint failure (F821 undefined name db in workflows.py) might be a pre-existing issue, not because of this PR.
This change only modifies frontend/src/pages/Scans.tsx and frontend/testing/unit/pages/Scans.failures.test.tsx.
The ToolConfigTimeout max value mismatch is also pre-existing.
I apologise for the large number of commits.

Screenshots for checks

Description: The screenshot shows the passing of all 5 tests.

Description: The screenshot shows the successful building after the running of the tests, generating no warnings.