Add RSS feed generation

[shigahi]

Summary

Add automatic RSS feed generation at /rss.xml endpoint.

Closes #30

Changes

Worker Code (src/code.ts)

• Add RssOptions interface with enabled, title, description, and language fields
• Add RSS configuration constants (RSS_ENABLED, RSS_TITLE, RSS_DESCRIPTION, RSS_LANGUAGE)
• Implement generateRssFeed() function that generates RSS 2.0 XML
• Implement escapeXml() helper function for proper XML escaping
• Add /rss.xml endpoint handler that returns the RSS feed when enabled

UI (src/App.tsx)

• Add RSS state management with useState<RssOptions>
• Add RSS configuration section in Advanced Settings:
  • Enable RSS toggle
  • Feed title input (defaults to domain)
  • Feed description input
  • Language selector (en-us, en-gb, ja, de, fr, es, zh-cn, zh-tw, ko)

RSS Feed Features

• RSS 2.0 compliant XML format
• Includes atom:link for self-reference
• Uses PAGE_METADATA for item titles and descriptions
• Proper XML entity escaping for special characters
• Correct Content-Type header (application/rss+xml; charset=utf-8)

Testing

• Local build passes (npm run build)
• Generated worker code contains RSS configuration
• RSS feed endpoint only responds when enabled