[Feature] Rich push formatting for daily digest

[jerry609]

Source TODO

• HTML 邮件模板
• Slack Block Kit 消息
• 钉钉 Markdown 优化

Scope

• Implement HTML email template
• Implement Slack blocks payload
• Improve DingTalk markdown layout

Acceptance Criteria

• Push output is structured and readable across channels

[jerry609]

Implemented in commit 1e8f0ad on branch feat/dailypaper-sse-stream: Slack Block Kit payload, improved DingTalk markdown layout, and coverage tests for push formatting.