feat[electrum]: Support server.features on all feature combinations

[junderw]

Closes #149

Adds the server.features endpoint on electrum regardless of feature flags.

If the electrum-discovery feature isn't enabled, the hosts value is an empty HashMap.

At least this will prevent errors when clients attempt to query this.

[Copilot]

Pull request overview

Adds unconditional support for the Electrum server.features RPC so Electrum/Sparrow clients don’t fail when electrum-discovery is not enabled, aligning behavior with the standard Electrum protocol expectations.

Changes:

• Make server.features available regardless of electrum-discovery, returning a shared ServerFeatures struct per connection.
• Always include electrum_public_hosts in Config, defaulting to None when electrum-discovery is disabled so hosts serializes as an empty map.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/electrum/server.rs	Removes feature-gating for server.features and wires a shared Arc<ServerFeatures> into each connection.
src/config.rs	Makes electrum_public_hosts part of Config in all builds, with None when discovery is not compiled in.

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[mononaut]

Tested ACK @ [15e9430]

tested with the various combinations of electrum-discovery and --electrum-public-hosts and confirmed this fixes the server.features RPC error and doesn't seem to break anything.