feat(settings): public_base_url field in general settings UI

roundtable/templates/settings/general.html

@@ -9,6 +9,18 @@
 <div class="card" style="max-width:640px;">
   <h2 class="section-title" style="margin-bottom:1.25rem;">General</h2>
 
+  <div class="form-group">
+    <label>Public base URL</label>
+    <input type="text" name="general.public_base_url"
+           value="{{ settings['general.public_base_url'] }}"
+           placeholder="https://monitor.example.com">
+    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.25rem;">
+      External URL where this Roundtable instance is reachable from the outside.
+      Leave blank to use whatever hostname you're currently browsing via.
+      Used by install scripts, share links, and alert notifications.
+    </div>
+  </div>
+
   <div class="form-group">
     <label>Session lifetime <span style="color:var(--text-muted);font-size:0.8rem;">(hours)</span></label>
     <input type="number" name="session.lifetime_hours" min="1" max="720"