feat: complete Zustand store migration to combined slices pattern

[danieloche635-bit]

Summary

Refactor ~25 individual Zustand stores into a single combined store using Zustand slices pattern. Improves modularity, cross-slice communication, performance, testability, and reduces bundle size.

Changes

Infrastructure

• Created 11 domain slice files in src/store/slices/
• Created src/store/combinedStore.ts with persist middleware (AsyncStorage)
• Created src/store/index.ts with backward-compatible named exports
• Added integration tests
• Added migration guide at docs/store-migration.md

Migration

• All individual store files re-export from combinedStore with deprecation notices
• Restored missing constant/type exports in webhookStore.ts, cancellationStore.ts, communityStore.ts
• Updated all consumer files to import from central store

Backward Compatibility

• All old import paths continue to work
• No breaking changes