fix: avoid WebUI ready banner when assets missing

[Soulter]

Summary

• Check dashboard static entry files before printing the WebUI ready banner
• Log a clear warning and start only the web backend/API path when WebUI assets are missing
• Add regression coverage for missing WebUI static files

Fixes #8795

Tests

• uv run ruff format .
• uv run ruff check .
• uv run pytest tests/test_dashboard.py::test_dashboard_ssl_missing_cert_and_key_falls_back_to_http tests/test_dashboard.py::test_dashboard_run_skips_ready_banner_when_static_files_missing

Summary by Sourcery

Guard the WebUI startup flow against missing static assets and adjust logging and readiness output accordingly.

Bug Fixes:

• Avoid printing the WebUI ready banner and implying a functional frontend when required static files are missing, while still starting the backend APIs.

Tests:

• Extend dashboard SSL fallback test to run with an explicit WebUI directory containing minimal assets.
• Add a regression test verifying that dashboard run logs a warning, starts only the web backend, and skips the WebUI ready banner when static files are missing.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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[gemini-code-assist]

Code Review

This pull request adds a check to verify if WebUI static files are present before starting the dashboard, falling back to starting only the backend APIs and skipping the ready banner if they are missing. Corresponding unit tests were also added. Feedback on the changes highlights a critical issue: skipping the ready banner entirely when static files are missing prevents the initial generated credentials from being printed, permanently locking the user out. It is recommended to customize the banner to display backend API details and credentials even when frontend assets are missing.

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[Cyrene2026]

我重新验证了一下：按照当前逻辑，只要 data/dist/index.html 存在，AstrBot 就会打印 WebUI is ready。
但我只是在 data/dist 里放了一个空的 index.html，没有 assets/，WebUI 仍然被判定为 ready。
这说明这里检查的并不是 WebUI 是否真的可用，而只是 index.html 是否存在。一个空文件都能通过，很显然，这里并不是真正检查 WebUI readiness check

[Soulter]

我重新验证了一下：按照当前逻辑，只要 data/dist/index.html 存在，AstrBot 就会打印 WebUI is ready。
但我只是在 data/dist 里放了一个空的 index.html，没有 assets/，WebUI 仍然被判定为 ready。
这说明这里检查的并不是 WebUI 是否真的可用，而只是 index.html 是否存在。一个空文件都能通过，很显然，这里并不是真正检查 WebUI readiness check

有谁会这样做。。