Refactor state handling to Zustand

[asturur]

Summary

Replace the current V2 state handling approach with Zustand-based stores.

Scope

• inventory the current state split across React context, refs, reducers, and ad-hoc effects
• define Zustand stores for app data, file/card data, selection, and editing state
• migrate consumers incrementally while preserving existing behavior
• remove obsolete context/reducer wiring once Zustand owns the state graph

Acceptance Criteria

• core V2 state is managed through Zustand stores
• state updates are easier to trace and test
• legacy context/reducer paths that are no longer needed are removed