feat(flutter/api): dio client with Bearer interceptor + ApiError

ApiError.fromDio handles both server envelope shapes: {"error":"code"}
(admin endpoints) and {"error":{"code","message"}} (most others). Same
dual-shape problem the web apiFetch already solves. Token comes from a
resolver function so the auth provider can swap implementations
without touching the client.

ErrorCopy is a lazy singleton over assets/error-copy.json with the
same fallback chain web uses (specific code -> unknown -> hard-coded).

flutter_client/lib/api/error_copy.dart

@@ -0,0 +1,22 @@
+import 'dart:convert';
+import 'package:flutter/services.dart' show rootBundle;
+
+class ErrorCopy {
+  ErrorCopy._(this._table);
+  final Map<String, String> _table;
+
+  static ErrorCopy? _instance;
+
+  static Future<ErrorCopy> load() async {
+    if (_instance != null) return _instance!;
+    final raw = await rootBundle.loadString('assets/error-copy.json');
+    final m = (jsonDecode(raw) as Map).cast<String, dynamic>().map(
+          (k, v) => MapEntry(k, v.toString()),
+        );
+    _instance = ErrorCopy._(m);
+    return _instance!;
+  }
+
+  String forCode(String code) =>
+      _table[code] ?? _table['unknown'] ?? 'Something went wrong.';
+}