feat: add typed selector hooks for app stores

[Meet-hybrid]

Description

Adds explicit, generic selector hooks for the messaging, notification, and volunteer Zustand stores. Components can now subscribe to only the state they need, for example useMessagingStoreSelector((state) => state.messages), avoiding re-renders from unrelated updates.

Includes regression tests that verify focused selectors do not re-render when unrelated store slices change.

Related Issue

Closes #902

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Validation

• pnpm vitest run src/app/store/__tests__/selectorHooks.test.tsx
• pnpm run type-check
• pnpm run lint

Checklist

• Code follows project style guidelines
• Self-review completed
• No console errors introduced

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