Preserve rich thread history

[olegberman]

Summary

• preserve sanitized formatting, tables, links, remote image references, and matching CID images in quoted replies
• collapse HTML and plain-text quoted history behind an accessible ellipsis
• collapse intervening thread messages behind a counted divider while keeping the first and final two visible
• prepare signed release 0.1.17

Root cause

The reply builder escaped only the selected message's plain-text body, so Gmail could recognize the quote wrapper but had no original HTML or inline MIME images to display. The reader also rendered quotes and every chronological message without disclosure state.

Validation

• CI=true pnpm check
• CI=true pnpm deploy:dry-run
• 187 unit tests
• 7 Worker integration tests