fix(scans): replace error banner with toast feedback for delete and rescan failures

[NaitikVerma6776]

Description

The Scans page was using setError() to display a persistent error banner for delete, clear-history, and bulk-delete failures, which was inconsistent with the rest of the app's non-blocking toast pattern. The rescan failure had no user feedback at all — only a console.error.
Changes:

• Added useToast import and hook to Scans.tsx
• Replaced all 3 setError() calls in catch blocks with addToasterror
• Added addToast feedback to the previously-silent rescan failure catch
• Updated existing test (handles clearAllTasks failure correctly) to reflect the new toast-based pattern
• Added ToastProvider wrapper to all 3 existing Scans test files so useToast no longer throws
• Added frontend/testing/unit/pages/ScansToastErrors.test.tsx with 4 focused tests covering the new toast behavior

Related Issues

Closes #1396

Type of Change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

All 28 tests across the full Scans test suite pass locally via npm test (Scans.test.tsx, Scans.polling.test.tsx, ScansPhases.test.tsx, ScansToastErrors.test.tsx).

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
• My changes generate no new warnings

[utksh1]

The direction of this fix is correct — replacing setError() with addToast() for failure feedback is the right approach. However, it's incomplete:

1. Dead error banner code was left behind:
After replacing all setError(...) calls in catch blocks with addToast(...), the error banner infrastructure is now orphaned:

• const [error, setError] = useState<string | null>(null) is still declared
• The entire {error && (<div role="alert" ...>) } JSX block is untouched
• Several setError(null) calls remain scattered (filter change, pagination, modal reset)

No catch block sets setError with a user-facing message anymore, making the banner dead code. Please remove:

• The error state declaration
• The {error && (...)} JSX block
• All setError(null) calls that are now no-ops

2. Minor: ScansToastErrors.test.tsx is missing a trailing newline.

Once the dead code is cleaned up, this is a clean, well-tested fix that's ready to merge.