feat(dx): add MSW for local development without a backend

[Vic-Soso]

Description

Local dev required a running Rust backend, which new contributors often could not run.

• lib/msw/handlers.ts: Mock Service Worker handlers for every api.* call in lib/api.ts (29 endpoints), with realistic seed data — a pre-authenticated demo merchant/wallet, a mix of confirmed/detected/failed payments across several assets and days (so the dashboard and feat(home): add 7-day revenue chart to the dashboard #494's revenue chart have something real to show), pending/paid payment requests, a completed withdrawal, KYC, an offramp order lifecycle, and SEP-24 deposit/withdraw responses.
• components/demo-mode-provider.tsx: starts the worker before rendering children when NEXT_PUBLIC_DEMO_MODE=true, so nothing (in particular SessionProvider's getSession call) can race the worker's readiness. A no-op otherwise — the mock worker code isn't even imported outside demo mode.
• README: new "Demo Mode (No Backend Required)" section under Quick Start.
• public/mockServiceWorker.js generated via npx msw init public/.

Also fixes (pre-existing, unrelated, but found while building this)

lib/msw/__tests__/handlers.test.ts calls every api.* method against the handler set through msw/node with onUnhandledRequest: 'error', which is what actually proves each method has a matching handler rather than falling through to a real network call. That test caught a real, previously-untested bug in lib/api.ts unrelated to MSW itself: stringifyWithBigInts's marker-stripping regex searched for a literal NUL byte in the JSON.stringify output, but JSON.stringify always escapes a raw NUL in a string value as a six-character text escape sequence — so the regex never matched, and every bigint field (amount_stroops on createPaymentRequest and createWithdrawal) went out over the wire as a quoted string wrapping that escape sequence around the digits instead of a bare JSON integer. This would have broken those two calls against the real backend too, not just the mocks. Fixed the regex and added lib/__tests__/api.test.ts as a dedicated regression test.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Related Issues

Closes #486

Testing

• lib/msw/__tests__/handlers.test.ts: exercises all 29 api.* methods against the handler set, asserting no request falls through unhandled
• lib/__tests__/api.test.ts: regression test for the stringifyWithBigInts fix, asserting the wire format is a bare JSON integer
• components/__tests__/demo-mode-provider.test.tsx: renders immediately when demo mode is off; shows a loading state → starts the worker → renders children when on; still renders children if the worker fails to start
• npm run type-check — no new errors from any file this PR touches
• npx eslint clean on all changed files; npx prettier --check clean

Checklist

• Tests pass
• Coverage maintained (≥70%) — see test files above; lib/msw/handlers.ts itself is fully exercised by the completeness test
• Documentation updated (README "Demo Mode" section)

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