test: add Playwright E2E regression workflow

[chainTe]

Summary

Adds a Playwright-based Electron E2E regression framework for Neuron and documents the functional regression scope for release testing.

Changes

• Add a prioritized Neuron wallet functional regression plan under e2e/.
• Add Playwright configuration and Electron fixture for launching packages/neuron-wallet with isolated user data.
• Add initial Electron smoke coverage for app startup and wallet entry routing.
• Add yarn test:e2e, headed, and UI scripts.
• Add a GitHub Actions E2E workflow that runs under Ubuntu + Xvfb and uploads Playwright report/test-result artifacts for screenshots, videos, and traces.

Validation

• npx playwright test --list
• npx yarn@1.22.22 build:main
• git diff --check

Full Electron E2E execution is expected to run in GitHub Actions via Xvfb.