feat(web): restore user prompt to composer input on revert

[MaximIJ]

What Changed

When reverting a checkpoint, the user's original prompt text is now restored into the composer input field so they can edit and re-send without retyping.

Specifically:

• onRevertUserMessage now looks up the reverted user message and extracts the visible prompt text using deriveDisplayedUserMessageState (strips terminal context blocks)
• onRevertToTurnCount accepts an optional promptToRestore param and writes it back into the composer after the revert completes
• A 300ms setTimeout defers the restore past the server state sync (throttled at ~100ms) to prevent re-renders from overwriting it
• The composer is focused with cursor at end of text after restore

Why

When a user reverts a message, they typically want to tweak the prompt and re-send. Currently they have to retype the entire prompt from memory. Restoring it into the composer removes that friction.

This is a small, focused UX improvement — one file changed, no new dependencies, no scope expansion.

Checklist

• This PR is small and focused
• I explained what changed and why
• No UI layout changes (behavior-only change, no screenshots needed)

Note

Restore user prompt to composer input after reverting to a message checkpoint

When a user reverts to a previous message, the visible prompt text of that message is now restored into the composer and focused, with the cursor placed at the end.

• onRevertUserMessage in ChatView.tsx looks up the target message and derives its visible text via deriveDisplayedUserMessageState before passing it to onRevertToTurnCount.
• On successful revert, onRevertToTurnCount schedules prompt restoration via setTimeout, updating promptRef, calling setPrompt, collapsing the composer cursor, and focusing the composer.

^{Macroscope summarized 8399fcf.}

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