feat: add deeplinking support for oauth login

lib/features/onboarding/providers/oauth_provider.dart

@@ -0,0 +1,94 @@
+import 'dart:io';
+
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+import 'package:vaani/api/api_provider.dart';
+import 'package:vaani/models/error_response.dart';
+
+import '../models/flow.dart';
+
+part 'oauth_provider.g.dart';
+
+/// the string state of a flow started by user
+typedef State = String;
+
+/// the verifier string of a flow started by user
+typedef Verifier = String;
+
+/// the code returned by the oauth provider
+typedef Code = String;
+
+@Riverpod(keepAlive: true)
+class OauthFlows extends _$OauthFlows {
+  @override
+  Map<State, Flow> build() {
+    return {};
+  }
+
+  void addFlow(
+    State oauthState, {
+    required Verifier verifier,
+    required Uri serverUri,
+    required Cookie cookie,
+    bool replaceExisting = false,
+  }) {
+    if (state.containsKey(oauthState) && !replaceExisting) {
+      return;
+    }
+    state = {
+      ...state,
+      oauthState: Flow(
+        state: oauthState,
+        verifier: verifier,
+        serverUri: serverUri,
+        cookie: cookie,
+        isFlowComplete: false,
+      ),
+    };
+  }
+
+  void markComplete(State oauthState, String? authToken) {
+    if (!state.containsKey(oauthState)) {
+      return;
+    }
+    state = {
+      ...state,
+      oauthState: state[oauthState]!
+          .copyWith(isFlowComplete: true, authToken: authToken),
+    };
+  }
+}
+
+/// the code returned by the server in exchange for the verifier
+@riverpod
+Future<String?> loginInExchangeForCode(
+  LoginInExchangeForCodeRef ref, {
+  required State oauthState,
+  required Code code,
+  ErrorResponseHandler? responseHandler,
+}) async {
+  final flows = ref.watch(oauthFlowsProvider);
+  final flow = flows[oauthState];
+  if (flow == null) {
+    throw StateError('No flow active for state: $oauthState');
+  }
+
+  if (flow.authToken != null) {
+    return flow.authToken;
+  }
+
+  final api = ref.read(audiobookshelfApiProvider(flow.serverUri));
+  final response = await api.server.oauth2Callback(
+    code: code,
+    codeVerifier: flow.verifier,
+    state: oauthState,
+    cookie: flow.cookie,
+    responseErrorHandler: responseHandler?.storeError,
+  );
+
+  if (response == null) {
+    return null;
+  }
+
+  ref.read(oauthFlowsProvider.notifier).markComplete(oauthState, api.token);
+  return api.token;
+}