fix(ui): shorten Freenet URLs in reply previews

[skandragon]

Replies quoted the sender's raw gateway URL (http://127.0.0.1:7509/v1/contract/web/<id>/...) where the message body already renders the short freenet:<id-prefix> label.

The body path goes through finalize_anchors; the reply-quote strip is plain text and goes through strip_markdown → collect_mdast_text, which had no Node::Link arm. Adding one there covers all three preview call sites at once — the rendered reply strip (:381) and the two composer previews (:5313, :5384).

Label-only: the strip has no href, so there is nothing to host-rewrite there. [label](url) text is left alone, mirroring the body's "visible text equals href" rule.

Not parity with the body, deliberately

finalize_anchors compares scraped HTML anchor text against the href after markdown's sanitize_uri percent-encoding; this arm compares raw mdast on both sides. A URL whose suffix markdown percent-encodes (non-ASCII, ^, |) therefore shortens in the preview but not in the body. One-way superset, and harmless — the preview is a non-clickable text node, so the worst case is a shortened label where the body kept the full URL, never the reverse. Documented at the arm.

Tests

clean_reply_preview_shortens_bare_freenet_urls covers: bare URL with path suffix, non-loopback host, trailing-slash normalisation, non-Freenet link passthrough, explicit [label](url), and a spoof URL that only mentions the marker in its query. Mutation-checked — removing the arm reddens two assertions, inverting the guard reddens three.

cargo test -p river-ui --bins: 892 passed.

Known gaps, all pre-existing and left alone

• Block boundaries run together (- <url>\n- x → freenet:UDzGbcWrx). The raw URL ran into the next item too; the label just makes it misleading rather than ugly. Fix belongs in the Paragraph/ListItem path.
• Node::Image alt text is dropped entirely.

No contract/delegate/common changes — no migration or redeploy needed.