Add per-file review collapse state to the diff panel

[BunsDev]

Summary

• add local per-file review state for diff selections in the web diff panel
• add collapse and accept controls for each rendered file section
• auto-expand a targeted file when the user opens it from the changed-files tree

Validation

• bun fmt
• bun lint
• bun typecheck
• cd apps/web && bun x vitest run src/lib/diffFileReviewState.test.ts