feat: send email notifications for missed messages

- New cron job in cluster
- Handle presence and other concerns
- Toggle in app to disable email notifications
- Handles other edge cases such as cooldown period
- Simple html email with simple message

Closes #117

js/src/api/client.ts

@@ -125,6 +125,12 @@ class ApiClient {
     return data.url;
   }
 
+  // --- Settings ---
+
+  async updateSettings(data: { email_notifications_enabled?: boolean }): Promise<void> {
+    await this.requestVoid("PATCH", "/humans/me/settings", data);
+  }
+
   // --- Depot ---
 
   async prepareUpload(data: PrepareUploadRequest) {