test: add frontend smoke coverage

[Sikkra]

Summary

Adds a focused React Testing Library smoke test for the frontend app so npm test -- --watchAll=false runs a real test suite instead of exiting with No tests found.

This covers the bridge shell, both conversion tabs, and the wallet connection guidance rendered through the existing MetaMask hook flow.

Verification

• npm install in app/
• CI=true npm test -- --watchAll=false in app/ (1 passed)
• git diff --check

Build note: npm run build on Node.js v24 still requires the legacy OpenSSL provider for this older react-scripts@4 stack; node --openssl-legacy-provider ./node_modules/react-scripts/scripts/build.js succeeds with existing warnings.

Related: #7
Program reference: BitgesellOfficial/bitgesell#81

USDT-TRC20 payout address if accepted: TPwPFww7zxXFQ7zugo22gktQhckWVarRqi

[MyTH-zyxeon]

Maintainer-facing review assist for the #81 bounty queue:

This is a narrow, useful testing PR: it turns the frontend from "No tests found" into a real React Testing Library smoke check that covers the bridge shell, both conversion tabs, and the unavailable-wallet guidance path.

Acceptance checks I would use before merge:

• Keep the MetaMask mock deliberately on status: 'unavailable', since that makes the test deterministic without requiring a wallet provider in CI.
• Run the stated clean checks from app/: CI=true npm test -- --watchAll=false and git diff --check.
• If maintainers require a build gate before bounty approval, use the already-noted legacy OpenSSL command for this older react-scripts@4 stack, rather than treating Node 24's default OpenSSL failure as a regression in this PR.
• Consider pairing this with fix: show BGL deposit submit errors #36 during review: test: add frontend smoke coverage #35 gives the frontend harness, while fix: show BGL deposit submit errors #36 adds a real rejected-submit behavior test.
• For bounty accounting, tie this explicitly back to BGL (Bitgesell) Bounty/Improvement Program ($50000 budget) bitgesell#81 after approval so submitter and maintainers agree on the review lane.

No payment action or claim from me here; just a focused review pass to help maintainers evaluate the PR.