Add AI-assisted commit message generator

## Summary\nCreate an AI-assisted commit flow that can draft commit messages based on the user\'s staged changes.\n\n## Requirements\n- Provide a UI entry point (button/modal) in the commit view to request an AI-generated message.\n- Gather the relevant git context that Gitui already tracks (branch name, staged diff per file, linked issue/branch metadata, optional user hint).\n- Serialize that context into a deterministic prompt (e.g., structured sections with fenced diff blocks) and send it to a backend proxy LLM endpoint.\n- Display the returned suggestion so the user can edit/accept it before committing.\n- Ensure network failures or empty responses gracefully fall back to the normal manual workflow.\n\n## Notes\n- The backend proxy already exists; Gitui only needs to assemble the prompt text/JSON and call it.\n- Keep request size reasonable—consider truncating or batching very large diffs.