chore: final code review cleanup before release

- Add missing Sanitize() calls in LoadAsync() backup-recovery and normal paths
- Remove unused System.Threading.Tasks import from App.axaml.cs
- Remove unused BuildUrl dead code from HttpUploadService
- Remove unnecessary 'new' keyword on AppLanguageManager.Instance
- Fix LoadAsync() to catch IOException/UnauthorizedAccessException

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: JusterZhu

src/App.axaml.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Threading.Tasks;
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.ApplicationLifetimes;

src/Configuration/ConfigService.cs

@@ -141,6 +141,7 @@ public async Task LoadAsync()
                 {
                     var backupJson = await File.ReadAllTextAsync(_backupPath);
                     Config = JsonConvert.DeserializeObject<AppConfig>(backupJson, JsonSettings) ?? new AppConfig();
+                    Config.Sanitize();
                     await SaveAsync(); // Restore main file from backup
                     return;
                 }
@@ -160,6 +161,7 @@ public async Task LoadAsync()
         {
             var json = await File.ReadAllTextAsync(_configPath);
             Config = JsonConvert.DeserializeObject<AppConfig>(json, JsonSettings) ?? new AppConfig();
+            Config.Sanitize();
 
             // Run schema migrations
             Migrate();

src/Lingua/AppLanguageManager.cs

@@ -25,7 +25,7 @@ namespace Irihi.Lingua;
 [LinguaManager("./Resources/Locales")]
 public sealed class AppLanguageManager : LanguageManager
 {
-    public static new AppLanguageManager Instance { get; } = new();
+    public static AppLanguageManager Instance { get; } = new();
 
     private AppLanguageManager() : base("zh-CN")
     {

src/Services/Upload/HttpUploadService.cs

@@ -190,13 +190,6 @@ private static bool TryBuildUrl(UploadConfig config, out string fullUrl, out str
         return true;
     }
 
-    [Obsolete("Use TryBuildUrl for validation.")]
-    private static string BuildUrl(UploadConfig config)
-    {
-        TryBuildUrl(config, out var url, out _);
-        return url;
-    }
-
     private static void ApplyAuth(HttpRequestMessage request, UploadConfig config)
     {
         var plain = AuthCredentialEncryptor.Decrypt(config.Auth);