Fix issues #168, #42, #44, #170

[Rampop01]

Fixes #168
Fixes #42
Fixes #44
Fixes #170

Changes Made

1. Sidebar Tests: Added comprehensive component tests for the Sidebar navigation component covering rendering, active states, route changes, responsive behavior, and keyboard accessibility.
2. Test Suite: Added an integration test for the dashboard overview page to ensure critical render paths are covered, fulfilling the requirement for a frontend test suite alongside existing tests.
3. axios.ts Audit: Refactored ApiError to extend the native Error class, standardizing error shapes for React Query hooks while preserving 401 redirect constraints.
4. merchService Tests: Confirmed comprehensive unit tests are in place for merchService (CRUD, pricing, and inventory operations).

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