chore: switch to anvim@0.1.2, remove local engine, add autosave, update tests

[bug-breeder]

This PR migrates the app to the published ANVIM package and cleans up redundant code.

Changes

• Replace in-repo ANVIM with npm package anvim@0.1.2; update imports in utils and tests
• Adapt tests to anvim@0.1.2 behavior (z-clear consumes 'z'; incremental composition)
• Vite test config: inline anvim dependency for Vitest resolution
• Editor: add autosave (localStorage + 500ms debounce), restore on load, clear on Clear, save on Paste
• Remove legacy browser extension (background, content script, popup, manifest)
• Update dependencies (package.json, yarn.lock)

Context

• Package: anvim@0.1.2 (npm)
• Package info: https://www.npmjs.com/package/anvim

Follow-ups

• Keep app tracking npm package for future engine updates