Persist diff view state across tab switches and reloads

[selfcontained]

Summary

• Persist collapsed files, collapsed directories, and scroll position in the Changes tab per agent via Jotai atomFamily backed by localStorage
• Scroll position is debounced (300ms) and restored once when landing on an agent's tab (survives page reloads and tab switches without fighting background refetches)
• Stale localStorage entries are cleaned up when agents are archived

Changed files

• apps/web/src/lib/store.ts — DiffViewState type, diffViewStateAtomFamily, reconcileDiffViewStateStorage
• apps/web/src/components/app/changes-tab.tsx — replaced ephemeral useState with atom-backed state, lifted collapsedDirs from FileTree, added scroll tracking/restore
• apps/web/src/hooks/use-media-sidebar-state.ts — added diff view state cleanup call

Test plan

• Type check (pnpm run check)
• Production build (pnpm run finalize:web)
• E2E tests (172 passed)
• Store unit tests (8 passed)
• Frontend UX review (approved after 2 rounds)
• Product review (approved after 2 rounds)
• Manual: collapse files, scroll, switch to Terminal and back — state preserved
• Manual: reload the page — collapse state and scroll position restored
• Manual: switch between agents — each agent has its own collapse/scroll state
• Manual: archive an agent — its localStorage entry is cleaned up