🐛 fix(update/proxy): 稳定更新检查与代理运行时

- 更新检查前恢复持久化全局代理，避免重启后代理状态未进入运行时

- 将 DNS、EOF 和临时网络异常转为本地化错误，并对可重试网络错误做短重试

- 优先读取 GitHub asset digest，缺失时再回退 SHA256SUMS 校验文件

- 新增全局代理连接测试后端接口，支持草稿代理、已保存密码复用和清除密码意图

- 更新 Wails 绑定、浏览器 mock 与代理草稿序列化测试

frontend/src/utils/globalProxyDraft.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest';
 
-import { createGlobalProxyDraft, toPersistedGlobalProxy } from './globalProxyDraft';
+import { createGlobalProxyDraft, toPersistedGlobalProxy, toSaveGlobalProxyInput } from './globalProxyDraft';
 
 describe('global proxy draft', () => {
   it('hydrates a secretless draft from backend metadata while keeping password input blank', () => {
@@ -32,4 +32,18 @@ describe('global proxy draft', () => {
     expect('password' in persisted).toBe(false);
     expect(persisted.hasPassword).toBe(true);
   });
+
+  it('passes explicit saved password clearing intent to the backend save payload', () => {
+    const input = toSaveGlobalProxyInput({
+      enabled: true,
+      type: 'socks5',
+      host: '127.0.0.1',
+      port: 7890,
+      hasPassword: true,
+      clearPassword: true,
+    });
+
+    expect(input.password).toBe('');
+    expect(input.clearPassword).toBe(true);
+  });
 });

frontend/src/utils/globalProxyDraft.ts

@@ -1,5 +1,9 @@
 import { GlobalProxyConfig } from '../types';
 
+type SaveGlobalProxyDraft = Partial<GlobalProxyConfig> & {
+  clearPassword?: boolean;
+};
+
 const toTrimmedString = (value: unknown): string => {
   if (typeof value === 'string') {
     return value.trim();
@@ -53,10 +57,11 @@ export function toPersistedGlobalProxy(value: Partial<GlobalProxyConfig> = {}):
   };
 }
 
-export function toSaveGlobalProxyInput(value: Partial<GlobalProxyConfig> = {}): GlobalProxyConfig {
+export function toSaveGlobalProxyInput(value: SaveGlobalProxyDraft = {}): GlobalProxyConfig & { clearPassword?: boolean } {
   const draft = createGlobalProxyDraft(value);
   return {
     ...draft,
     password: typeof value.password === 'string' ? value.password : '',
+    clearPassword: value.clearPassword === true || undefined,
   };
 }