disable delete button when no layers selected

Added updateButtonStates() to enable/disable delete button based on selection
Updated control setup and selection handlers to call this method
Added CSS for disabled button state and tooltip
Delete button now disables when no layers are selected; all other panel features unchanged

src/css/layers_panel.css

@@ -51,6 +51,17 @@
     background: #5a5a5a;
 }
 
+.layers-btn:disabled {
+    background: #2a2a2a;
+    color: #666666;
+    cursor: not-allowed;
+    opacity: 0.5;
+}
+
+.layers-btn:disabled:hover {
+    background: #2a2a2a;
+}
+
 .layers-container {
     flex: 1;
     overflow-y: auto;