fix: restore frontend source code (src/web/) that was accidentally deleted

Frontend source is needed for development/building the SPA.
Only Node.js runtime server code should be removed.

src/web/i18n.test.ts

@@ -0,0 +1,47 @@
+import { describe, expect, it } from 'vitest';
+import { translateText } from './i18n.js';
+
+describe('translateText', () => {
+  it('keeps zh text unchanged in zh mode', () => {
+    expect(translateText('模型广场', 'zh')).toBe('模型广场');
+  });
+
+  it('translates exact key in en mode', () => {
+    expect(translateText('模型广场', 'en')).toBe('Model Marketplace');
+  });
+
+  it('supports phrase replacement for mixed text', () => {
+    expect(translateText('覆盖槽位 3', 'en')).toBe('Coverage Slots 3');
+    expect(translateText('共 12 个模型', 'en')).toBe('Total 12 models');
+  });
+
+  it('never returns Chinese characters in strict en mode', () => {
+    const samples = [
+      '站点已禁用',
+      '缓存清理后重建失败：unknown error',
+      '签到任务执行中，请稍后查看签到日志',
+    ];
+
+    for (const sample of samples) {
+      expect(translateText(sample, 'en')).not.toMatch(/[\u3400-\u9fff]/);
+    }
+  });
+
+  it('uses concrete english translations instead of fallback for common runtime text', () => {
+    expect(translateText('切换到中文', 'en')).toBe('Switch to Chinese');
+    expect(translateText('中', 'en')).toBe('ZH');
+
+    const samples = [
+      '站点已禁用',
+      '签到任务执行中，请稍后查看签到日志',
+      '下游访问令牌至少 6 位（含 sk-）',
+      '路由重建任务执行中，请稍后查看程序日志',
+    ];
+
+    for (const sample of samples) {
+      const translated = translateText(sample, 'en');
+      expect(translated).not.toBe('Untranslated');
+      expect(translated).not.toMatch(/[\u3400-\u9fff]/);
+    }
+  });
+});