show rules of the correct instance on the auth screen (#4358)

closes #4357

app/src/main/java/com/keylesspalace/tusky/components/login/LoginWebViewViewModel.kt

@@ -39,7 +39,7 @@ class LoginWebViewViewModel @Inject constructor(
         if (this.domain == null) {
             this.domain = domain
             viewModelScope.launch {
-                api.getInstance().fold(
+                api.getInstance(domain).fold(
                     { instance ->
                         _instanceRules.value = instance.rules.map { rule -> rule.text }
                     },

app/src/main/java/com/keylesspalace/tusky/network/MastodonApi.kt

@@ -89,7 +89,9 @@ interface MastodonApi {
     ): NetworkResult<InstanceV1>
 
     @GET("api/v2/instance")
-    suspend fun getInstance(): NetworkResult<Instance>
+    suspend fun getInstance(
+        @Header(DOMAIN_HEADER) domain: String? = null
+    ): NetworkResult<Instance>
 
     @GET("api/v1/filters")
     suspend fun getFiltersV1(): NetworkResult<List<FilterV1>>