fix: correct empty-state copy on Host Overview page

Since RTT history keys make host.plugin_data non-empty after the very
first heartbeat, the "no hosts" empty state can now only trigger when
zero hosts have ever connected, not when hosts exist but haven't sent
plugin metrics yet (that in-between state no longer exists). Update
the copy to describe the actual current trigger condition.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DEimzMv4Q5EjFg3hoiZ69T

hbd/server/templates/plugins.html

@@ -290,8 +290,8 @@
 
       {% if not hosts %}
       <div class="no-data">
-        <p>No hosts with plugin data available</p>
-        <p style="font-size:0.9em;margin-top:10px;">Hosts will appear here once they start sending plugin metrics</p>
+        <p>No hosts have connected yet</p>
+        <p style="font-size:0.9em;margin-top:10px;">Hosts will appear here once they start sending heartbeats</p>
       </div>
       {% else %}
       <div id="hosts-container">