feat(testing): add reusable Playwright browser journeys

[gbaudrit]

Summary

Add product-owned reusable Playwright journeys and make their screenshots stable across Console screens by rendering computed sticky elements in normal document flow only while a capture is taken.

Changes

• add the Playwright workspace, managed product hosts, page objects, reusable authentication and agent-creation journeys, and capture CLI
• support capture URL overrides for external consumers
• normalize every computed sticky element during both page and target screenshots, then restore the DOM in a finally block
• cover target and full-page capture modes with a browser regression test
• document the screenshot behavior

Validation

• TypeScript type check succeeds: npm exec tsc -- --noEmit
• Playwright suite passes: 5 passed
• Agent creation browser scenario passes with system Chrome
• Focused Web tests pass: 8 passed
• Desktop smoke test completed against the local executable
• Mobile smoke test not run; the capture normalization is viewport-independent
• No unnecessary breaking change introduced

Breaking changes

None.