docs(web): document per-action progress callbacks

[yaodong-shen]

Summary

• document how to observe each atomic aiAct action and capture a Playwright screenshot before execution continues
• add AiActProgressEvent and isAiActProgressEvent() so consumers can narrow the generic progress stream without any or unsafe casts
• re-export the progress types and guard from both @midscene/web and @midscene/web/playwright
• keep the English and Chinese Playwright guides in sync, including failure-site capture and listener cleanup

Closes #2794.

Validation

• pnpm exec nx test @midscene/core (123 files, 1350 passed, 8 skipped)
• pnpm --filter @midscene/core exec vitest --run tests/unit-test/agent-progress-bus.test.ts (8 passed)
• pnpm exec nx run-many --target=build --projects=@midscene/core,@midscene/web
• pnpm exec nx build doc (including search metadata checks)
• repository install/prepare full build (27 projects)
• biome check . --diagnostic-level=info --no-errors-on-unmatched --fix (1386 files, clean)
• repository pre-commit lint-staged and commitlint hooks

Notes

The progress listener is awaited by the existing progress bus. The documented callback therefore captures the page after action_done or at the action_failed failure site, before the next action begins or aiAct() rejects.