feat(comments): add review-level comments across review scope

[villads-valur]

@agavra I keep running into line comments and file comments not being enough for my reviews, so I added a Review Comment type.

I.e. if you want the LLM to look at all the type casts in your PR:

1. **[ISSUE]** `Review Comment (scope: PR diff origin/main..HEAD)` - There are many explicit type casts in this PR, please reduce unnecessary casts and make the underlying types more precise.

Summary

• add review-level comments that are not tied to a specific file or line, including add/edit/delete support in the app state
• render a Review Comments section at the top of unified and side-by-side diff views, plus a shortcut (;c) and help/status text updates
• include review-level comments in markdown export using an explicit Review Comment (scope: ...) location label

Validation

• rtk cargo check
• rtk cargo test

[agavra]

Tested it locally and the feature makes sense to me. Thansk @villads-valur - I'll cut a release today, enough good new features :)