feat: add scan history sidebar to Findings page

[aaniya22]

Fixes #276

Users had no way to browse or reload past scan results from the UI.

Changes:

• Added ScanHistory component that fetches past tasks from the existing /api/v1/tasks endpoint and lists them in a sidebar
• Wired the sidebar into the Findings page — clicking a past scan loads its findings via getTaskResult()
• Active scan is highlighted in the sidebar

No new backend endpoints needed — reuses existing /tasks and /task/{id}/result APIs.

[utksh1]

Thanks for the Findings scan-history idea. This needs changes before merge: it introduces a separate history sidebar inside Findings that duplicates the Scans page/navigation, uses design tokens/classes that do not match the current SecuScan styling, includes a replacement-character separator in the UI, and replaces the findings list with task-result data without preserving existing filters/review state/loading/error handling. Please align it with the existing design/data flow and add focused tests for selecting a scan.

[aaniya22]

Thanks for the Findings scan-history idea. This needs changes before merge: it introduces a separate history sidebar inside Findings that duplicates the Scans page/navigation, uses design tokens/classes that do not match the current SecuScan styling, includes a replacement-character separator in the UI, and replaces the findings list with task-result data without preserving existing filters/review state/loading/error handling. Please align it with the existing design/data flow and add focused tests for selecting a scan.

done