contact messages: visitor panel and composer

The composer is a constrained contenteditable (plain-text paste,
toolbar formatting, upload chips for images/attachments) serialized to
a Markdown subset by richtext.js — the wire format is text and the
server re-renders it, so no user markup ever reaches a page. Uploads go
to /_contact/files with the session token (admins share the component
via /_api/files). ContactPanel shows the thread pushed live over the
activity socket: pagerite.js lazy-loads the bundle on trigger clicks,
routes socket pushes to it, re-registers the session token on every
reconnect (the resync), and dots trigger buttons on unread replies.

frontend/src/assets/pagerite.css

@@ -878,6 +878,37 @@ article {
   opacity: 1;
 }
 
+/* {contact: Label} directive: the button opening the visitor contact
+   panel (docs/messages.md). A standalone accent button in the article
+   flow; .has-unread (set by contact-main) marks a new admin reply. */
+.contact-trigger {
+  position: relative;
+  padding: 0.35em 1.1em;
+  font: inherit;
+  color: var(--accent);
+  background: none;
+  border: 1px solid var(--accent);
+  border-radius: 1.2em;
+  cursor: pointer;
+}
+
+.contact-trigger:hover {
+  color: var(--bg);
+  background: var(--accent);
+}
+
+.contact-trigger.has-unread::after {
+  content: "";
+  position: absolute;
+  top: -0.25em;
+  right: -0.25em;
+  width: 0.6em;
+  height: 0.6em;
+  border-radius: 50%;
+  background: var(--accent);
+  box-shadow: 0 0 0 0.15em var(--bg);
+}
+
 .edit-link {
   position: absolute;
   top: 0.2rem;