Refactor: State persistence via Zustand persist middleware + projectStorage

[jpeggdev]

Problem

`useStatePersistence` (67 lines) hardcodes knowledge of 3 specific stores (uiStore, feedStore, endpointStore), their specific state shapes, and the `PersistedState` interface. Adding a new persisted field requires editing the hook, the interface, and both save/restore logic paths.

Additional issues:

• Single shared debounce timer — a high-frequency store (feed filters) resets the timer for low-frequency ones (panel layout)
• Single monolithic localStorage key per project — a corrupt `feedFilters` corrupts `activeSection`
• No schema versioning — shape changes on deploy silently corrupt or discard state
• Restore logic calls store internals directly (`setActiveSection`, `setSidebarCollapsed`, etc.)
• Zoom persistence in uiStore bypasses the system entirely with raw localStorage calls

Proposed Interface

~20 lines wrapping Zustand's built-in `persist` middleware with a dynamic project-scoped key:

// src/shared/lib/persistence.ts

export function projectStorage<T>(baseKey: string): PersistStorage<T>
// Resolved key at runtime: ads:project:{projectId}:{baseKey}

export const globalStorage: PersistStorage<unknown>
// For non-project-scoped state (zoom, theme)

Usage — adding persistence to a store is one middleware addition:

export const useFeedStore = create<FeedState>()(
  persist(
    (set, get) => ({ /* ...store unchanged... */ }),
    {
      name: 'feed-filters',
      storage: projectStorage('feed-filters'),
      partialize: (s) => ({ filters: s.filters }),  // persist only filters
    }
  )
)

Project switching triggers rehydration:

// In projectStore.switchProject:
await useFeedStore.persist.rehydrate()
await useUiStore.persist.rehydrate()

`useStatePersistence` hook is deleted entirely.

Dependency Strategy

• Category: In-process + local-substitutable (localStorage)
• `projectStorage` reads `useProjectStore.getState().activeProjectId` at read/write time
• Each store owns its own persistence config — no central hook knows about all stores
• Zustand's `persist` middleware handles serialization, debouncing, and hydration
• Each slice gets its own localStorage key — isolation prevents cross-corruption
• `projectStore` itself uses `globalStorage` or no persistence (avoids circular dependency)

Testing Strategy

• New boundary tests: Create store with `persist` + `projectStorage`, set state, switch project ID, verify new key written. Verify `partialize` excludes ephemeral state. Verify `rehydrate()` restores correct project's state.
• Old tests to delete: Any tests for `useStatePersistence` hook (deleted entirely)
• Test environment: Standard Vitest; can use in-memory storage adapter for Zustand persist

Implementation Recommendations

• Each store should declare what to persist via `partialize` — adding a field is one line
• `projectStorage` should hide the dynamic key computation and project ID lookup
• `globalStorage` should be a thin alias for naming consistency
• Export a `rehydrateProjectStores()` helper listing all project-scoped stores, called from project switch
• The `version` option on Zustand persist handles schema migration (discard on mismatch, or use `migrate` callback for complex cases)
• Zoom persistence in uiStore migrates from raw localStorage to `globalStorage` + `partialize`
• The hook `useStatePersistence` is deleted — 67 lines removed, zero lines of orchestration code remain