Encode dots in URL path segments to prevent browser resolution

[DL6ER]

What does this implement/fix?

When editing a regex domain that is just "." (single dot), the browser
interprets it as a relative path component ("current directory") and
resolves it away before sending the request. The server receives an
empty string, causing "Invalid request: Specify item in URI".

This happens because encodeURIComponent(".") returns "." unchanged —
dots are unreserved characters per RFC 3986. The browser only normalizes
literal "." and ".." path segments, not percent-encoded "%2E".

Add utils.encodePathSegment() which wraps encodeURIComponent and also
encodes dots to %2E, then apply it in groups-domains.js (the reported
bug) and groups.js (which was also missing encoding entirely).

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Related issue or feature (if applicable): Fixes #3308

Pull request in docs with documentation (if applicable): N/A

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[yubiuser]

I tried it and still get a 400 response.