Set up Playwright end-to-end test suite

[Marvell69]

There are no end-to-end tests. A Playwright suite covering the core user journeys (connect wallet, submit invoice, fund invoice, mark paid) provides the highest-value regression safety net before mainnet.

Requirements and context

• Install and configure Playwright
• Mock wallet connection using Playwright's extension injection or a test wallet
• E2E tests: connect wallet, submit an invoice (testnet), view invoice detail, fund invoice (LP), mark paid (payer)
• Run against a local development environment with mocked contract
• Integrate into CI (ci.yml) — fail on any E2E failure
• Include screenshots in CI artifacts on failure

Suggested execution

git checkout -b test/playwright-e2e-suite

• Install Playwright and configure for Next.js
• Create wallet mock/injection utility
• Write core journey tests
• Add to CI workflow

Example commit message
test: set up Playwright E2E suite for core user journeys