Keep confirmed post replies visible across chat turns

[16francej]

Keeps confirmed post replies visible across chat turns, combining #874 with the new-chat fix from #990 while leaving reconnect redesign separate.

• Verified in Firefox with real model/core/web/portal turns: three consecutive replies hidden before, visible exactly once after.
• Failed-refresh, fork, navigation-race, and multiple-post regressions covered; 1,042 web tests pass.
• Typechecks, ESLint, Oxlint, formatting, and UI build pass.

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