Add empty-state messages to overview page

Grid sections for Racks and Devices now show a dashed 'No X yet' message
when the collection is empty, using Go template {{else}} blocks. CSS
styles the placeholder with muted text and a dashed border.

static/style.css

@@ -185,3 +185,6 @@ section { margin-bottom: 2em; }
 
 /* TOOLBAR */
 .toolbar { margin-bottom: 1em; }
+
+/* EMPTY STATE */
+.empty-state { padding: 2em 1em; text-align: center; color: var(--text-muted); font-style: italic; border: 1px dashed var(--border); border-radius: 4px; grid-column: 1 / -1; }

templates/overview.html

@@ -11,6 +11,8 @@
     <span>{{.RackType}} / {{.Depth}} / {{.HeightUnits}}U</span>
     {{if .Comment}}<small>{{.Comment}}</small>{{end}}
 </a>
+{{else}}
+<div class="empty-state">No racks yet. Create one below.</div>
 {{end}}
 </div>
 <details>
@@ -33,6 +35,8 @@
     {{if .Model}}<span>{{.Model.Manufacturer}} {{.Model.Name}}</span>{{end}}
     {{if .UsageDescription}}<small>{{.UsageDescription}}</small>{{end}}
 </a>
+{{else}}
+<div class="empty-state">No devices yet. Create one or add a rack first.</div>
 {{end}}
 </div>
 <details>