feat(relationships): explain the community's identifier default on the request form (#241 follow-up)

[stormer78]

Follow-up to #241 / #335.

#335 made a new relationship's mint default follow the working community's declared relationshipIdentifierDefault, but the form's identifier toggle was still static — Pairwise R-DID (recommended) / Your persona DID. So when a community declares attributed, the toggle silently started on the persona DID while pairwise still read "(recommended)", with no explanation.

This carries the declaration into the NewRequest form state and surfaces it:

• the toggle labels the persona-DID option "(your community's default)" and drops "(recommended)" from pairwise when the community prefers attributed (pairwise then reads "(more private)");
• a line under the toggle explains where the default came from — "Your community publishes relationships under members' persona DIDs, so this defaults there" or "Your community recommends a pairwise DID (the default)" — and nothing extra when no community declared one.

The lookup that seeded the default is refactored to return the declaration itself (working_community_relationship_default) rather than a bare bool, so the form gets both the default and the reason from one place. No behaviour change when no community declares a preference.

Tests

• the_form_explains_a_community_relationship_default — the form shows the right label + reason for attributed / pairwise, and nothing extra for none.
• The feat(relationships): honor a community's relationshipIdentifierDefault (#241) #335 default test updated to the new lookup and to assert the form mode it produces.
• cargo fmt, clippy --all-targets --all-features, RUSTDOCFLAGS="-D warnings" cargo doc, full cargo test all pass.