fix(library): preserve grid during background refreshes

[yicheng47]

Summary

• keep the library grid mounted during sync, MCP, cover, import, and book-mutation refreshes while preserving every loaded page
• move sync-progress state into Sidebar and memoize BookGrid with a stable change callback
• add hook regression coverage for foreground loading, silent replacement, and multi-page window preservation

Fixes #304

Test plan

• pnpm test
• npx tsc --noEmit
• pnpm run lint
• pnpm build