Show diff hunks for PR review threads

[KalLee-SI]

Summary

• fetch diffHunk for PR review comments
• render review threads as grouped activity blocks instead of flattening each reply
• show the thread location, diff hunk, and outdated marker in the PR activity tab

Fixes #852

Verification

• go test ./internal/tui/components/prview ./internal/data
• git diff --check
• git diff --cached --check

[ysrfz]

Hey @CalLeeLQY — heads up, I opened #891 with a broader take on #852 that overlaps with this one (thread grouping + hunks + a few other things from the issue). Happy to consolidate however the maintainers prefer.

[dlvhdr]

Hey @KalLee-SI, I think we should discuss the design first in an issue.
Let me know if you have time for that.