OAuth Support (#53)

Rips out the old auth method, adds OAuth support. Fixes #51.

Tested Platforms:
- [x] Linux
    - [x] Flatpak
    - [x] Native (Nix)
- [x] Android
- [ ] Windows
- [ ] MacOS

Reviewed-on: Nexus/nexus#53

lib/controllers/client_id.dart

@@ -0,0 +1,33 @@
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:nexus/controllers/client.dart";
+
+class ClientIdController extends AsyncNotifier<String> {
+  final Uri homeserver;
+  ClientIdController(this.homeserver);
+
+  @override
+  Future<String> build() => ref
+      .watch(ClientController.provider.notifier)
+      .registerClient(
+        .new(
+          clientName: "Nexus",
+          applicationType: .native,
+          grantTypes: .new([.authorizationCode, .refreshToken]),
+          responseTypes: .new([.code]),
+          logoUri: Uri.https(
+            "nexus.federated.nexus",
+            "raw/branch/main/assets/mobile.svg",
+          ),
+          homeserverUrl: homeserver,
+          clientUri: Uri.https("nexus.federated.nexus"),
+          redirectUris: .new([
+            .new(scheme: "nexus.federated.nexus", path: "/"),
+          ]),
+        ),
+      );
+
+  static final provider =
+      AsyncNotifierProvider.family<ClientIdController, String, Uri>(
+        ClientIdController.new,
+      );
+}