fix: preserve attachments in completed chat history

[16francej]

Preserve file references when rebuilding chat history so attached images and documents remain visible after completion and reload.

• Use the existing lossless transcript fallback for attachment results; no migration or frontend change.
• Verified the failure and fix through the real Pi harness and a local desktop/mobile browser build with deterministic model responses.
• Regression coverage includes multiple files, failed/repeated attaches, bounded history, and participant visibility.
• 290 focused core tests and 1,018 web tests pass; core typecheck, focused lint, and formatting pass.

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