feat(assistant): add failed-message retry and rate-limit countdown

[vitorhugo-dotnet]

Implements the frontend portion of vitorhugo-dotnet/SpringBoot-JobApplyTracker#87.

What changed

• consumes structured assistant SSE errors
• sends and persists the backend-required conversationId
• marks the exact originating user message as failed
• shows failure and retry actions beside the failed user message
• retries the existing message/response pair without duplicating the user message
• clears failed state after a successful retry
• honors provider-provided retry countdowns and keeps retry manual
• keeps retry immediately available when no delay is supplied
• persists retry state and migrates the previous history format
• stops the countdown timer once retry becomes available

Verification

• TDD RED confirmed before implementation
• Playwright: 73 tests passed ✅
• production TypeScript/Vite build ✅

Backend companion: vitorhugo-dotnet/SpringBoot-JobApplyTracker#95