Add thread section management APIs

[copyberry]

Add thread section management APIs

What changed

• Add threadSection/create, threadSection/update, and threadSection/delete app-server methods, with generated protocol schemas and TypeScript bindings.
• Persist custom sections in SQLite with stable UUIDv7 identities, trim and validate display names, and prevent renaming or deleting the built-in pinned section.
• Delete sections transactionally and return their active and archived threads to the unsectioned list.
• Serialize section mutations with section listing so concurrent requests observe ordered updates.

Testing

• Cover section persistence across restarts, ordered renames, member cleanup on deletion, invalid requests, unsupported stores, and thread-store/state-runtime behavior.