v4: what the server and the client both state is declared once — several rules were written out in two or more files, some already drifting apart. A link's site name (its host without www.) was computed in five places, now shared/url.ts's siteHost; dumpFileVersion, which the frontend and the server had to keep equal by hand, is one function in shared/dump.ts; the Markdown flattening of the chat's reply previews and the digest's quotes is shared/markdown.ts's markdownToPlainText, so the digest also keeps an image's alt text; an attachment's URL and how to find one in a text are shared/attachment.ts, which the editor, Markdown and attachment-service read, its id pattern the new UUID_RE that the file and thumbnail routes check ids with. On the server, the Bandcamp, YouTube playlist and link preview caches use one TtlCache (api/lib/ttl-cache.ts) instead of three copies of the same expiry and eviction; the link previews' theme-color and the emails' theme tokens read colours with one parser (api/lib/css-color.ts), and a transparent theme-color now gives no tint; the WebSocket route sends through ws-service's send, the welcome included, so it is type-checked; fetch.ts strips an IPv6 literal's brackets with public-address's bareHostname; and a category's slug is checked by the same validateSlug as a dump's, with its own length. The migrations keep their frozen copies, as they document. 272 backend tests and 677 frontend tests pass, and deno check, tsc, eslint and doc:lint are clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

shared/url.ts

@@ -0,0 +1,28 @@
+/**
+ * Names the site a link points at, as the app shows it: a link dump's title
+ * when its page has none, a preview's site name, the digest's domain line and
+ * the tint of a card without artwork. The server's link checks key on it too
+ * (`canonical-url.ts`), so a link reads as the same site everywhere.
+ *
+ * @module
+ */
+
+/**
+ * Returns `url`'s host without a leading `www.`; `URL` has already lowered
+ * its case. `www.youtube.com` and `youtube.com` give `youtube.com`.
+ */
+export function siteHostOf(url: URL): string {
+  return url.hostname.replace(/^www\./, "");
+}
+
+/**
+ * Returns {@link siteHostOf} the URL `raw`, or undefined when `raw` isn't
+ * one.
+ */
+export function siteHost(raw: string): string | undefined {
+  try {
+    return siteHostOf(new URL(raw));
+  } catch {
+    return undefined;
+  }
+}