feat(aggregator): add source name field and improve formatting
- Add source_name field to Perspective model for better attribution
- Extract source name from HTML anchor tags in parser
- Display source name in rich text (e.g., "The Straits Times" vs generic "Source")
- Improve spacing in highlights, perspectives, and sources lists (double newlines)
- Better visual separation between list items
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Co-Authored-By: Claude <noreply@anthropic.com>