add invite support

Closes #82

lib/helpers/extensions/join_room.dart

@@ -0,0 +1,71 @@
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:material_ui/material_ui.dart";
+import "package:nexus/controllers/client.dart";
+import "package:nexus/controllers/sidebar_page.dart";
+import "package:nexus/helpers/extensions/focus_room.dart";
+import "package:nexus/models/requests/join_room.dart";
+
+extension JoinRoom on BuildContext {
+  /// Joins a room, showing progress, success (with a button to open the room)
+  /// and errors in snackbars.
+  ///
+  /// Returns the joined room's ID, or null if joining failed.
+  Future<String?> joinRoom(
+    JoinRoomRequest request, {
+    required String name,
+  }) async {
+    final container = ProviderScope.containerOf(this);
+    final messenger = ScaffoldMessenger.of(this);
+    final colorScheme = Theme.of(this).colorScheme;
+
+    final joining = messenger.showSnackBar(
+      .new(content: Text("Joining $name..."), duration: const .new(days: 1)),
+    );
+
+    try {
+      final roomId = await container
+          .read(ClientController.provider.notifier)
+          .joinRoom(request);
+
+      joining.close();
+      messenger.showSnackBar(
+        .new(
+          content: Text("Joined $name."),
+          action: .new(
+            label: "Open",
+            onPressed: () async {
+              if (await container.focusRoom(roomId)) {
+                container
+                    .read(SidebarPageController.provider.notifier)
+                    .set(.rooms);
+              } else {
+                messenger.showSnackBar(
+                  .new(
+                    content: Text(
+                      "$name hasn't finished syncing yet, try again in a moment.",
+                    ),
+                  ),
+                );
+              }
+            },
+          ),
+        ),
+      );
+
+      return roomId;
+    } catch (error) {
+      joining.close();
+      messenger.showSnackBar(
+        .new(
+          backgroundColor: colorScheme.errorContainer,
+          content: Text(
+            error.toString(),
+            style: .new(color: colorScheme.onErrorContainer),
+          ),
+        ),
+      );
+
+      return null;
+    }
+  }
+}