feat: Add dashboard extension hook for third-party plugins (v0.4.6)

Add wp_prometheus_register_dashboards action hook allowing third-party
plugins to register their own Grafana dashboard templates.

- DashboardProvider: registration system with file/JSON support
- Security: path traversal protection, JSON validation
- Admin UI: "Extension" badge and plugin attribution
- Isolated mode support for dashboard registration hook

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/css/admin.css

@@ -216,6 +216,32 @@
     font-size: 13px;
 }
 
+/* Third-party dashboard card styling */
+.wp-prometheus-dashboard-card.third-party {
+    position: relative;
+    border-color: #2271b1;
+}
+
+.wp-prometheus-dashboard-card .dashboard-badge {
+    position: absolute;
+    top: -8px;
+    right: -8px;
+    background: #2271b1;
+    color: #fff;
+    font-size: 10px;
+    font-weight: 600;
+    padding: 3px 8px;
+    border-radius: 10px;
+    text-transform: uppercase;
+    letter-spacing: 0.5px;
+}
+
+.wp-prometheus-dashboard-card .dashboard-plugin {
+    color: #646970;
+    margin: -5px 0 15px 0;
+    font-style: italic;
+}
+
 /* Import options panel */
 #import-options {
     border-radius: 4px;