replaced account data singleton with a session model

lib/utils/random_id.dart

@@ -0,0 +1,19 @@
+import 'dart:math';
+
+/// Random hex id from a cryptographic RNG, e.g. for per-install identifiers.
+String randomHexId({int bytes = 16}) {
+  final random = Random.secure();
+  return List<int>.generate(
+    bytes,
+    (_) => random.nextInt(256),
+  ).map((b) => b.toRadixString(16).padLeft(2, '0')).join();
+}
+
+/// Random RFC 4122 version-4 UUID, for ids a server expects in UUID form.
+String randomUuidV4() {
+  final hex = randomHexId();
+  final variant = (int.parse(hex[16], radix: 16) & 0x3 | 0x8).toRadixString(16);
+  return '${hex.substring(0, 8)}-${hex.substring(8, 12)}-'
+      '4${hex.substring(13, 16)}-$variant${hex.substring(17, 20)}-'
+      '${hex.substring(20)}';
+}