feat: add HTML template rendering and test notification functionality

templates.json

@@ -16,5 +16,9 @@
   "reminder_halftime": {
     "color": 15105570,
     "text": "Half-time in 5 minutes!"
+  },
+  "test": {
+    "color": 3447003,
+    "text": "This is a test notification."
   }
 }