Use React Query for checkpoint diff loading and caching

- add shared provider checkpoint diff query options and keys
- refactor `ChatView` and `DiffPanel` to fetch diffs via React Query
- persist selected-turn checkpoint diffs back into thread state for reuse
8307207485Julius Marminge committed on 2/18/2026, 9:09:57 AMparentd35ca86
9 files changedLine totals unavailable