fix(prompts): localize web report item labels for English mode

[nankingjing]

Problem

buildWebReportPrompt in src/prompts.ts uses hardcoded Chinese labels for item fields, regardless of the lang parameter. When English reports are generated (lang="en"), the prompt leaks Chinese text:

• Item field labels: 分类, 发布/更新, 内容节选, 未知
• Empty-section message: 暂无可供分析的内容
• Section wrapper uses full-width Chinese parentheses （...）

This happens inside the siteSections builder (lines 593-606), which runs before the language-branching return statement. The lang parameter is available but not consulted for these strings.

Fix

Add English-localized strings for the three affected locations, guarded by the lang parameter:

1. Empty-items message — provides an English alternative: (mode, no content available for analysis.)
2. Item field labels — uses Category/Published/Updated/Unknown/Excerpt when lang="en"
3. Section header parens — uses standard ASCII parentheses (...) for English

Verification

• Reproduction script (run with npx tsx): before fix, English output contained Chinese characters; after fix, clean English.
• No behavioral change for the default lang="zh" path.
• TypeScript: npx tsc --noEmit passes (verified locally).

[nankingjing]

Reviewed — itemLabels i18n is cleanly structured, covers all label keys and the empty/no-content messages. Ready for review.

[nankingjing]

Re-reviewed and fixed the section header spacing: the Chinese lang="zh" path now keeps the original full-width parentheses without an extra leading space, while English uses ASCII parentheses with a leading space. Local tsc --noEmit, eslint src, and focused prompt-output tests pass; CI run is still blocked pending workflow approval.