Persist changed-files expansion state per thread

[Marve10s]

What changed

• Persist changed-files tree expansion state per thread instead of keeping it inside MessagesTimeline component state.
• Move the expansion source of truth into the web store, keyed by scoped thread id and turn id.
• Clean up stored expansion overrides when threads disappear.

Why

The changed-files tree reset to expanded whenever the thread view unmounted and mounted again. That happened because the expansion map lived inside MessagesTimeline as local React state.

Notes

• Default behavior stays the same: changed-files trees start expanded.
• Only collapsed overrides are stored; expanding again removes the override.
• This adapts the upstream t3code#1858 behavior to DPcode's current store structure.

Validation

• bun fmt
• bun lint
• bun typecheck
• bun run --cwd apps/web test --run src/store.test.ts src/components/chat/MessagesTimeline.test.tsx