Download report feature implemented

[kckDeepak]

Summary

• Adds a "Download Report" button to Stage 4 (Step4Report.vue) that appears once report generation is complete
• Wires it to the existing backend endpoint GET /api/report/<report_id>/download which was already implemented but
  never exposed in the UI
• Adds downloadReport() to frontend/src/api/report.js using responseType: 'blob' so axios handles the file
  response correctly

Why

Users completing the full 5-stage pipeline had no way to save or share the generated report. The backend already had a
working download endpoint returning the assembled full_report.md — the UI just never called it.

Changes

File	Change
frontend/src/api/report.js	Added downloadReport(reportId) function
frontend/src/components/Step4Report.vue	Added download button, handleDownload() handler, and button CSS

Test Plan

• Run the full 5-stage pipeline to completion
• Confirm "Download Report" button appears in Stage 4 when report is complete
• Click the button — browser should download {report_id}.md
• Open the file and confirm it contains the full formatted report
• Confirm "Go to Interaction" button still works normally

Closes #636