Improve API explorer UI and add Playwright E2E

[JedimEmO]

Summary

• rewrite generated REST and JSON-RPC API explorer UIs with dark-default static templates
• add dedicated Playwright fixture servers and browser E2E coverage
• add Playwright E2E job to the existing GitHub Actions CI workflow

Verified locally

• cargo fmt --check
• cargo test -p ras-rest-macro
• cargo test -p ras-jsonrpc-macro
• cargo check -p playwright-rest-fixture -p playwright-jsonrpc-fixture --all-features
• cargo test --workspace --all-features --no-run --locked
• npm ci && npm test in tests/playwright
• headed slow Playwright run: 9 passed