Web session preflight: Chromium-only gate and local-agent conflict warning

Two UX preconditions for any WebHID session:

- Feature-detect WebHID and degrade with an explanation on Safari/Firefox — both have declined the API.
- Warn when local software (an OpenLogi agent or Options+) likely holds the receiver: two open handles on one node is the known "replies reach only one of the two" failure, so the page must tell the user to quit the local software first. Detection is best-effort (a probe request that times out).

No files or tests are named. Start at the WebHID session entry point, then verify the Chromium feature gate and best-effort probe timeout; done means unsupported browsers receive an explanation and users are warned to quit OpenLogi or Options+ when the receiver appears occupied.

Written by the indexing model from the issue text.