Check the availability of Firebase before accepting new tokens

src/data/api_client.rs

@@ -9,4 +9,10 @@ pub struct APIClient {
     pub comment: Option<String>,
     pub default_expiration_time: u64,
     pub firebase_token: Option<String>,
+}
+
+impl APIClient {
+    pub fn is_firebase_available(&self) -> bool {
+        self.firebase_token.is_some()
+    }
 }