Enhance compare view with detailed breakdown and export options

RequestCompareModal:
- Add text diff view with side-by-side line comparison (LCS algorithm)
- Show system prompt and tools in diff, not just messages
- Add size breakdown: system prompt, tools, messages in KB
- Show cache read/creation tokens separately
- Add message size (KB) to each message row in structured view
- Add download options: .diff, .json, .md formats
- Add "Side-by-Side" export for external diff tools
- Toggle between Structured and Text Diff views

_index.tsx:
- Fix cache display to only show when > 0

Author: ejc3