feat(reports): add report comparison view (closes #36)

[Pragati5-DEBUG]

Adds a frontend-only report comparison view at /reports/compare so you can diff findings between two scans without exporting PDFs.

• Baseline vs comparison dropdowns load from GET /reports and GET /findings
• Diff sections: new findings, fixed findings, unchanged, severity changed (matched by title + target + category)
• Failed scans with findings appear in the dropdowns (labeled (scan failed)), not only status: ready
• Reports page: “Compare Reports” link; normalizeReport() avoids blank list when API omits assets
• Tests: compareFindings.test.ts (6), ReportCompare.test.tsx (2)

Closes #36.

Scope

Area | Change -- | -- frontend/src/utils/compareFindings.ts | Diff logic frontend/src/pages/ReportCompare.tsx | Compare UI frontend/src/pages/Reports.tsx | Link + report normalization frontend/src/routes.ts, App.tsx | Route /reports/compare

No backend API changes.

How to test

1. Start backend + frontend; set API key in Settings.
2. Run at least two scans that produce findings (ready or failed).
3. Open Reports → Compare Reports (or /reports/compare).
4. Confirm both dropdowns list scans; failed ones show (scan failed).
5. Pick baseline (older) and comparison (newer); verify four diff sections update.
6. Pick the same report in both dropdowns → “Select two different reports”.
7. cd frontend → npm test -- --run testing/unit/utils/compareFindings.test.ts testing/unit/pages/ReportCompare.test.tsx

Notes

• Comparison uses stored findings per task_id, not report PDF export.
• If dropdowns are empty, you need two reports that each have findings (or API/auth failure).

[utksh1]

Reviewed the report comparison route, finding diff utility, failed-report inclusion, and frontend tests. The feature is self-contained and acceptable for merge after updating against current main.