refactor: consolidate error handling between strandsParser and ChatPanel

[ShotaroKataoka]

Background

PR #118 added agent-level error display to fix the infinite spinner issue.
The implementation works correctly but has some structural improvements to make.

Tasks

• Extract error classification into a shared utility (e.g. classifyAgentError(message))
  • Returns error type: conversation_limit | throttling | timeout | not_ready | service_unavailable | generic
  • Single source of truth for string matching logic
• Move UI display messages out of strandsParser.js into the UI layer
  • Parser returns error type/code, ChatPanel decides display text
• Unify error display behavior: message replacement (not concatenation to currentCompletion)
• (Nice to have) Add error_code field to backend SSE error events to reduce reliance on string matching

Context

• strandsParser.js: 5 error categories with emoji-prefixed messages
• ChatPanel.tsx: 3 categories (retryable / conversation-limit / generic)
• Current concatenation: currentCompletion + errorMessage may produce awkward output if partial response exists

Related

• fix(web-ui): surface agent-level errors in chat UI #118