Add Relays group (chat/file/message) + faded homepage dividers

New collapsible 'Relays' group in the sidebar (chat/file/message relay) that
expands to show its items and persists open state. Homepage gets a matching
Relays card group, and areas are now split by faded separator bars like the
sidebar. Relay pages are placeholder 'coming soon' stubs (/relays/{kind}).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

manager/templates/relay.html

@@ -0,0 +1,15 @@
+{% extends "base.html" %}
+{% block title %}{{ title }} — SimpleX Manager{% endblock %}
+
+{% block content %}
+<div class="flex-between" style="margin-bottom:24px;">
+  <h1 style="margin:0;">{{ title }}</h1>
+  <a href="/" class="muted" style="text-decoration:none;">← Home</a>
+</div>
+
+<div class="card" style="text-align:center;padding:48px 24px;">
+  <div style="font-size:40px;line-height:1;margin-bottom:12px;">🔀</div>
+  <strong>{{ title }} — coming soon</strong>
+  <p class="muted" style="margin-top:8px;">This relay isn’t implemented yet.</p>
+</div>
+{% endblock %}