fix: preserve Chat thinking block order

[klioen]

Summary

• preserve Pi assistant content block order across thinking, text, and tool calls
• use stable block-level IDs so streaming updates replace blocks without moving them
• keep restored snapshots and live event timelines consistent
• preserve visible provider errors when no text block exists

Root cause

SpireCode aggregated every assistant text block into one message and inserted that message before iterating thinking blocks. The same message-first ordering was used for live events, so the reducer correctly retained the wrong initial position.

Validation

• red regression test reproduced text -> thinking output from thinking -> text source
• focused Chat tests: 3 files, 20 tests passed
• pnpm check: 63 files, 384 tests passed; 18 release tests passed
• git diff --check