Add frontend component tests

[akonopcz]

Overview

Extend frontend test coverage by adding component tests using React Testing Library.

Current state: 6% coverage (utility tests only)
Target: 30%+ coverage

Components to Test

High Priority

• SoundButton.tsx - Rendering, click handler, loading states (~5 tests)
• CategoryTabs.tsx - Tab switching, active state, rendering (~5 tests)
• DashboardHeader.tsx - Rendering, props (~3 tests)

Medium Priority

• Settings.tsx - Tab navigation (~3 tests)
• AudioDeviceSettings.tsx - Device selection, volume controls (~5 tests)
• PlaybackSettings.tsx - Toggle states (~3 tests)

Technical Requirements

• Mock Context Providers (AudioContext, SettingsContext, SoundLibraryContext)
• Extended Tauri API mocks
• React Testing Library patterns

Related

• Follows up on Integration test suite #37 (Integration Tests)
• Vitest setup already complete

Acceptance Criteria

• Component tests for high priority components
• Frontend coverage >= 30%
• All tests passing in CI