fix: persist chain error detail and move error banner to bottom

[Zeptiny]

Errors from failed chat turns are now persisted on the chain row (error_detail, error_title) so they survive session switches and restarts. The ErrorBanner moved from the top to the bottom of the chat stream, the Failed badge on ChainFooter shows the error as a tooltip, and hydrating a session with a FAILED chain restores the error banner automatically.

• Add errorDetail/errorTitle to Chain type, ChainStorageDict, DB schema (with migration for existing DBs), and storage R/W
• Thread error fields through persistTurnConversation → persistTurn → finishActiveChain at error emission time
• Populate lastChainError in chat:snapshot and session:open payloads
• Restore error via local_error projection on hydration
• Move ErrorBanner below streamNodes in ChatStream
• Add errorDetail tooltip to ChainFooter Failed badge
• Update all Chain construction sites and test factories

[coderabbitai]

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