fix: XSS prevention and frontend cleanup

[ChinmayShringi]

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

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Original PR: 666ghj/MiroFish#231
Original Author: @0xNyk