Improve error handling and typing in merchStore class

[coderabbitai]

Description

Enhance the merchStore class in src/entities/merch/model/store.ts with better error handling and type safety.

Tasks

• Update loadMerchCatalog method to return an empty array instead of undefined when errors occur or when the request is not successful
• Add proper TypeScript typing to the class and its methods
• Ensure consistent error handling patterns across the store

Context

This issue was created as a follow-up to review comments on PR #889 that were valid but out of scope for that feature implementation.

References

• PR: DRAFT: 883-feat: Make swipe gallery for merch card #889
• Discussion: DRAFT: 883-feat: Make swipe gallery for merch card #889 (comment)
• Requested by: @ansivgit