test: add unit tests for useAuthStore (setAuth/logout/persist)

[Xaxxoo]

Summary

• Add comprehensive unit tests for the useAuthStore Zustand store in src/lib/store.ts
• Tests cover setAuth (updates token/merchant in store and persists to localStorage), logout (clears state and removes legacy key), persist middleware (state survives simulated reload), and clearLegacyAccessTokenKey helper

Test plan

• Run npm test and verify all new tests pass
• Verify setAuth writes both token and merchant to the Zustand persist key
• Verify logout clears state and legacy localStorage key
• Verify persist middleware round-trips state through localStorage

Closes #127

Generated with Claude Code

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