fix: XSS prevention and frontend cleanup

[0xNyk]

Summary

• Add DOMPurify dependency and create sanitize.js utility
• Sanitize all v-html content through renderMarkdown() in Step4Report and Step5Interaction
• Guard all setInterval calls with clearInterval to prevent timer leaks in Step3Simulation, MainView, SimulationRunView

Test plan

• Verify frontend builds: cd frontend && npm run build
• Test report and interaction views render markdown correctly
• Verify no duplicate timers accumulate during navigation

[0xNyk]

Verification

All checks pass on merged code:

Test	Result
sanitize.js exists with DOMPurify wrapper	✅ PASS
sanitizeHtml used in Step4Report (2 refs)	✅ PASS
sanitizeHtml used in Step5Interaction (2 refs)	✅ PASS
clearInterval guards in Step3Simulation	✅ PASS
clearInterval guards in MainView	✅ PASS
npm run build succeeds	✅ PASS

✓ 678 modules transformed.
✓ built in 1.80s