Prevent preference render storms

[ctoth]

Summary

• migrate every React preferences consumer from whole-store subscriptions to exact domain or primitive selectors
• add Zustand selector subscriptions for imperative consumers and prevent unrelated preference writes from triggering autolog pruning, media updates, output updates, or server sync checks
• debounce preference persistence while flushing the latest pending state on pagehide

Root cause and impact

The monolithic preferences store exposed one broad subscription surface. Components and services subscribed to the entire state, so a write in any preference domain rerendered unrelated UI and invoked unrelated service work. Rapid slider changes also synchronously serialized every intermediate store state.

This keeps the existing store actions and persisted shape while isolating subscribers to the values they actually consume. Preference bursts now coalesce into one storage write without losing the final state when the page is discarded.

TDD and validation

• RED: selector listeners were unsupported, three rapid updates produced three synchronous storage writes, and a general-preference change triggered autolog pruning
• GREEN: focused affected suites passed
• npm run typecheck
• npm test (105 files, 1,047 tests)
• npm run lint (passes; existing warnings remain)
• git diff --check

Closes #82