MBA-1433: Add frontend web UI for portfolio management (mocked data)

[devin-ai-integration]

Summary

Adds a frontend-only web UI under frontend/ (React 18 + TypeScript + Vite, react-router-dom) modernizing the CICS/BMS screens — all data is mocked; no COBOL sources touched. Jira: MBA-1433

Key design points:

• src/api/: typed ApiClient interface + in-memory MockApiClient (simulated latency), so a real backend can replace the mock without UI changes. types.ts mirrors the copybooks (PORTFLIO, POSREC, TRNREC, HISTREC) — e.g. Transaction.type: 'BU'|'SL'|'TR'|'FE', Portfolio.status: 'A'|'C'|'S'.
• src/validation/portvald.ts ports the PORTVALD.cbl rules exactly (PORTnnnn id, 10-digit non-zero account, type ∈ {STK,BND,MMF,ETF}, ±9999999999999.99 amount) with matching PORTVAL.cpy error messages; covered by Vitest unit tests.
• Screens: stubbed login with admin/read-only role guards (sessionStorage session), dashboard (replaces MENMAP), portfolio CRUD with delete confirmation, position inquiry with Prev/Next paging (PF7/PF8), transaction history with date-range filter + 10-row pages, transaction entry with client-side validation and computed amount, four read-only reports (RPTPOS00/RPTAUD00/RPTSTA00/RTNANA00), system status, plus ERRMAP-style error banners, toasts, and a global error boundary.

Run: cd frontend && npm install && npm run dev (see frontend/README.md). Lint, npm run build, and vitest run all pass.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/8725bad5e57d4f519746e4e3f714f0d5
Open in Devin Desktop: https://app.devin.ai/desktop/session/8725bad5e57d4f519746e4e3f714f0d5?variant=devin

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