decision: inline GENERATION_ERROR on the assistant bubble vs #49 error blocks + toast

Extracted from #39, which marks the streaming assistant message status=ERROR with an inline error-details disclosure. Main solved the underlying mislabeling differently: #40 routes failures to error messages and #49 renders enriched error blocks plus a View Logs toast. #49 does not cover mid-generation failures attached to the assistant bubble, so there is a real gap to discuss.

Pick one error surface (or define how the two compose) before implementation; the implementation is tracked separately.

**Tier**: 1 (decision)