fix: reuse shared csvExport utility in ActivityHistory export (#1266)

[Blessed-Femi]

Summary

Closes #1266

ActivityHistory had its own ad-hoc CSV export that joined cells with a bare , and duplicated the blob/anchor-click download mechanics — while frontend/src/utils/csvExport.ts already provided a shared, properly-escaping implementation (convertArrayToCSV / downloadCSV). This PR removes the duplicated logic and routes the dashboard activity export through the shared utility.

Problem

• CSV rows were built with [headers, ...rows].map((e) => e.join(",")).join("\n") — no escaping of commas, double quotes, or newlines.
• Any field containing a comma or quote would shift columns and corrupt the exported file when opened in spreadsheet apps.
• The download mechanics (Blob creation, hidden anchor click) were duplicated from csvExport.ts.

Changes

• frontend/src/components/dashboard/ActivityHistory.tsx
  • exportToCSV now maps events to row objects (same columns as before: Stream ID, Event Type, Amount, Timestamp, Tx Hash) and calls the shared downloadCSV(rows, filename).
  • Removed ~25 lines of duplicated Blob/anchor-click code and the unescaped .join(",").
  • Output format preserved: ISO timestamps, formatted amounts, empty string for missing tx hash, flowfi_activity_<timestamp>.csv filename.
• frontend/src/components/dashboard/ActivityHistory.test.tsx (new)
  • Verifies the export calls the shared downloadCSV with the expected row data and filename.
  • Verifies a field containing commas and double quotes yields a correctly quoted/escaped, spreadsheet-safe CSV via the shared convertArrayToCSV.
  • Verifies the export button is disabled when there are no events.

Acceptance criteria

• Exporting activity containing commas/quotes in any field produces a correctly escaped, spreadsheet-safe CSV — covered by the new component test plus the existing csvExport.test.ts escaping tests.

Verification

• vitest run — 33 test files / 289 tests passing.
• eslint on changed files — 0 errors.
• tsc --noEmit — no new type errors (the 26 pre-existing errors in unrelated test files are unchanged).

Fixes #1266