add support for creating rooms

Fixes #81

lib/widgets/start_dm_sheet.dart

@@ -0,0 +1,77 @@
+import "package:flutter_hooks/flutter_hooks.dart";
+import "package:material_ui/material_ui.dart";
+import "package:nexus/helpers/extensions/create_room.dart";
+import "package:nexus/widgets/sheet_layout.dart";
+
+/// Starts a DM, asking for a user ID unless [userId] is given. Pops with true
+/// if a DM was started.
+class const StartDmSheet({final String? userId, final String? name, super.key})
+    extends HookWidget {
+  @override
+  Widget build(BuildContext context) {
+    final userIdController = useTextEditingController();
+    final text = useValueListenable(userIdController).text.trim();
+    final target = userId ?? text;
+    final valid = RegExp(r"^@[^:\s]+:[^\s]+$").hasMatch(target);
+    final encrypted = useState(true);
+    final reasonController = useTextEditingController();
+
+    void submit() {
+      if (!valid) return;
+      context.startDirectMessage(
+        target,
+        name: name ?? target,
+        encrypted: encrypted.value,
+        inviteReason: reasonController.text.trim().isEmpty
+            ? null
+            : reasonController.text.trim(),
+      );
+      Navigator.of(context).pop(true);
+    }
+
+    return SheetLayout(
+      title: const Text("Start a Direct Message"),
+      actions: [
+        TextButton(
+          onPressed: Navigator.of(context).pop,
+          child: const Text("Cancel"),
+        ),
+        TextButton(
+          onPressed: valid ? submit : null,
+          child: const Text("Start"),
+        ),
+      ],
+      children: [
+        if (userId == null)
+          TextField(
+            controller: userIdController,
+            autofocus: true,
+            keyboardType: .emailAddress,
+            onSubmitted: (_) => submit(),
+            decoration: .new(
+              labelText: "User ID",
+              hintText: "@user:example.com",
+              errorText: text.isEmpty || valid ? null : "Enter a valid user ID",
+            ),
+          )
+        else
+          Text(
+            "Are you sure you want to start a direct message with ${name ?? userId}?",
+          ),
+        TextField(
+          controller: reasonController,
+          textCapitalization: .sentences,
+          onSubmitted: (_) => submit(),
+          decoration: const .new(labelText: "Invite reason (optional)"),
+        ),
+        SwitchListTile(
+          secondary: const Icon(Icons.lock),
+          title: const Text("Encrypted"),
+          subtitle: const Text("Can't be turned off later"),
+          value: encrypted.value,
+          onChanged: (value) => encrypted.value = value,
+        ),
+      ],
+    );
+  }
+}