Validate directory uri

[thibmeu]

Adds a dedicated /debug page for Web Bot Auth debugging.

Changes

• Moves directory validation off the main explainer page.
• Adds /debug with:
  • key directory validation
  • fetched directory JSON display
  • autofill from directory validation into signature verification
  • local signature-header verification UI
• Keeps directory proxy logic out of index.ts.
• Fixes /.well-known/http-message-signatures-directory response signing.
• Makes Turnstile rendering conditional so missing site key does not break /debug.

Test

• npm run test -w verification-workers -- --run
• npm exec prettier -- --check ...
• npm exec eslint -- ...

Closes #37

For the reviewer

While the behaviour mayn appear to be similar to #83, the implementation is quite distinct. The client uses a proxy only to fetch the directory data. All informations are still validated locally. We don't overload the index page. We also require a captcha to arbitrarily fetch an external directory