Add an edit action for the latest user message

[Amer-alsayed]

Fixes #762

What Changed

Adds an edit action to the latest eligible user message. Choosing edit restores that message into the composer, rehydrates image attachments when possible, and reverts the thread to the checkpoint immediately before that turn so the user can resend an updated prompt.

Why

This keeps the workflow close to Codex-style prompt editing without mutating historical messages in place. It reuses the existing checkpoint revert model instead of introducing a separate history-editing path.

UI Changes

Before:

After:

Validation

• bun run --filter=@t3tools/web typecheck
• bun run --filter=@t3tools/web test

Scope

• Single file change
• No new backend command
• Reuses existing checkpoint revert flow

Note

Add an edit action for the latest user message in the chat timeline

• Adds an Edit last message button (pencil icon) on the last user message eligible for revert in MessagesTimeline, replacing the generic Undo button for that specific message.
• On click, onEditLastUserMessage reverts the thread to just before agent work, then pre-populates the composer with the original message text and any restorable image attachments.
• Image attachments are restored by fetching their previewUrl and reconstructing File objects via the new restoreComposerImageAttachment helper; attachments that fail to reload trigger a thread error notice.
• onRevertToTurnCount is refactored to return Promise<boolean> so callers can gate composer restoration on revert success, and isRevertingCheckpoint is now reset in a finally block.

^{Macroscope summarized a98fca3.}

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