feat(ui): display LLM retry and failure error in modal

[JamesDAdams]

Summary

• Adds the error message payload to the chat.llm_retry WebSocket protocol message when an LLM call fails during backoff retries.
• Displays a "Show error" button in the retry indicator pill and "Show error details" button on definitive retry failures.
• Opens an "LLM Error" modal that pretty-prints formatted JSON error bodies with syntax highlighting and line numbers.
• Lifts the modal state to MessageList to prevent the modal from auto-closing when retry attempts begin or tick.

AI-Enhanced Development

Tell what models helped shape this PR:

• AI Models: Gemini 3.7 Flash

Cache Impact

Does this PR affect anything cached — system prompts, tool definitions, skills, or other context?

• No