Validate main registry/query URLs against library instance list

[tkuhn]

Summary

• Cross-check NANODASH_MAIN_REGISTRY / NANODASH_MAIN_QUERY against the nanopub library's discovered instance list (NanopubServerUtils.getRegistryServerList() / QueryCall.getApiInstances()) instead of trusting the env var blindly. When the env value isn't in the list, fall back to the first known instance and log a warning.
• Cache the resolved URLs for the JVM lifetime (double-checked locking) and warm them at startup via Utils.initMainUrls() from WicketApplication.init(), so the (potentially slow) discovery is paid once at boot instead of on the first user request.
• Drop the now-redundant Utils.getMainRegistryUrl() argument passed to PublishNanopub.publish(...) in PublishForm and PreviewPage — let the library use its own default.

Resolution rules

• Env var set and matches a library instance → use it (validated).
• Env var set, library list empty → use it unvalidated (warn).
• Env var set but no match → warn and fall back to first library instance.
• Env var unset, library list non-empty → use first library instance.
• Env var unset, library list empty → built-in default (warn).

URL comparison is normalised (trim, strip trailing slash, lowercase); results are returned with a trailing slash.

Test plan

• Boot nanodash with no NANODASH_MAIN_* env vars; confirm logs report library-discovered URLs and the app reaches the registry.
• Boot with NANODASH_MAIN_REGISTRY set to a known library instance; confirm "validated against library instance list" log line.
• Boot with NANODASH_MAIN_REGISTRY set to a bogus URL; confirm warn log and fallback to first library instance.
• Publish a nanopub via the publish form and via the preview page; confirm both succeed without the explicit registry URL.

🤖 Generated with Claude Code

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🎉 This PR is included in version 4.28.0 🎉

The release is available on:

• nanodash-4.28.0
• GitHub release

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