feat(flutter): 增加目录加载诊断与前台重试

lib/core/config/app_config.dart

@@ -80,9 +80,17 @@ class AppConfig {
     Future<bool> Function(String baseUrl)? probe,
     String? fallbackBaseUrl,
   }) async {
-    final candidateProbe = probe ?? _probeBaseUrl;
     for (final candidate in _probeCandidates) {
-      if (await candidateProbe(candidate)) {
+      final result = probe == null
+          ? await _probeBaseUrl(candidate)
+          : await _runInjectedProbe(candidate, probe);
+      if (kDebugMode) {
+        final status = result.ok
+            ? 'ok'
+            : 'fail(${result.reason ?? 'request failed'})';
+        debugPrint('[probe] $candidate -> $status');
+      }
+      if (result.ok) {
         applyRuntimeBaseUrl(candidate);
         return;
       }
@@ -90,25 +98,43 @@ class AppConfig {
     applyRuntimeBaseUrl(fallbackBaseUrl ?? defaultBaseUrl);
   }
 
-  static Future<bool> _probeBaseUrl(String baseUrl) async {
+  static Future<_ProbeResult> _runInjectedProbe(
+    String baseUrl,
+    Future<bool> Function(String baseUrl) probe,
+  ) async {
+    try {
+      return _ProbeResult(ok: await probe(baseUrl));
+    } catch (error) {
+      return _ProbeResult(ok: false, reason: error.toString());
+    }
+  }
+
+  static Future<_ProbeResult> _probeBaseUrl(String baseUrl) async {
     final client = HttpClient()..connectionTimeout = _probeTimeout;
     try {
       return await _requestCategories(
         client,
         Uri.parse('$baseUrl/app/categories'),
       ).timeout(_probeTimeout);
-    } catch (_) {
-      return false;
+    } catch (error) {
+      return _ProbeResult(ok: false, reason: error.toString());
     } finally {
       client.close(force: true);
     }
   }
 
-  static Future<bool> _requestCategories(HttpClient client, Uri uri) async {
+  static Future<_ProbeResult> _requestCategories(
+    HttpClient client,
+    Uri uri,
+  ) async {
     final request = await client.getUrl(uri);
     final response = await request.close();
     await response.drain<void>();
-    return response.statusCode == 200;
+    final ok = response.statusCode == 200;
+    return _ProbeResult(
+      ok: ok,
+      reason: ok ? null : 'HTTP ${response.statusCode}',
+    );
   }
 
   /// 后端返回的是相对路径（/files/...），这里补上服务地址。
@@ -118,3 +144,10 @@ class AppConfig {
     return '$apiBaseUrl$path';
   }
 }
+
+class _ProbeResult {
+  const _ProbeResult({required this.ok, this.reason});
+
+  final bool ok;
+  final String? reason;
+}