Add option to clean up resolved comments

## Summary

There's currently no easy way to clean up resolved comments/annotations in the Changes/Diff view. Once a comment thread is resolved, it remains visible and clutters the review surface, especially on larger PRs or after multiple review passes.

## Proposed

Add an action to bulk-clean up (hide or remove) resolved comments. Some options worth considering:

- A "Clear resolved comments" button in the Changes/Diff view toolbar
- A filter toggle to hide resolved threads (with the option to bring them back)
- A per-file or whole-PR scope

## Why

- Keeps the review surface focused on open/unresolved feedback
- Reduces visual noise after iterating on a PR
- Matches behavior users expect from other code-review tools

Happy to discuss the exact UX (hide vs. delete, persistence across sessions, etc.).

Start with the Changes/Diff view described in the issue and clarify the intended UX for resolved comment cleanup. Decide whether the action hides or removes threads, its scope and persistence, then verify that open feedback remains visible and resolved comments can be restored if hiding is chosen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.