Add redo support to Vim composer history

[copyberry]

Add redo support to Vim composer history

What changed

• Add a bounded redo stack for complete composer drafts, including paste payloads and image attachments. Ctrl+R now reapplies the last undone edit in Vim normal mode, while new edits clear stale redo history.
• Add the configurable vim_normal.redo action to the keymap schema and picker. Preserve reverse prompt-history search in Vim insert mode and non-Vim mode, and honor explicitly configured bindings.
• Keep pending edit snapshots outside the shared undo/redo byte budget so canceled commands cannot evict committed history.

Testing

• Cover grouped edits, Vim searches, direct draft changes, history limits, empty redo stacks, and custom or unbound shortcuts.

[github-actions]

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