Add AI-generated commit messages

[danipen]

Summary

Adds AI-generated commit messages to the Changes tab: a provider-agnostic AI layer in the main process, a settings pane to configure a provider/model/key, and composer controls to generate a commit message from the staged diff.

What's included

• AI engine (src/main/ai/) — provider registry, HTTP client, commit-context builder, and prompt construction, with encrypted credential storage (cipher.ts + store.ts).
• IPC surface — new ai handlers wired through shared/types.ts → shared/ipc.ts → preload → main/ipc.
• Renderer — AiComposerControls in the commit composer to generate/insert messages, plus an AiPane in Settings to pick provider, model, and API key.
• Icons & styling — new AI icon and features/ai.css.
• Docs — docs/ai-features.md documenting setup and usage.
• Tests — unit coverage for providers, prompt building, credential store, and the renderer AI lib.

Notes

Signing, provider keys, and all git/network work stay in the main process; the renderer talks to it only through the IPC contract.