fix: use cache key for side-by-side toggle instead of clearing cache

[pablospe]

Summary

Follow-up to #75 — implements the cache key approach as suggested.

• Include side-by-side mode in the diff cache key (path vs path:sbs) so both renders are cached separately — toggling with s is instant after the first render
• Invalidate cache on resize since diffs are width-dependent

Test plan

• Toggle side-by-side with s on a file, verify it re-renders
• Toggle back, verify it's instant (cache hit)
• Navigate to another file, come back, verify cache still works
• Resize the terminal, verify diff re-renders at the new width