Add unit tests for ThemeEngine

[alimaandev]

Summary

desktop/src/core/ThemeEngine.ts implements the theme mode state (dark/light), persistence via localStorage (key \riday-theme), and emits a \ heme:change\ event through EventBus. It currently has no test file.

Goal

Create desktop/src/test/ThemeEngine.test.ts covering:

• Default mode is \dark\
• \set()\ persists the choice to localStorage
• \get()\ returns the stored mode after a reload
• \set()\ emits a \ heme:change\ event via EventBus
• Invalid stored values fall back to \dark\

Tasks

• Write the Vitest test file (jsdom environment)
• Mock localStorage and EventBus where needed
• Follow the style of existing tests in desktop/src/test/

Acceptance criteria

• All listed behaviors are covered
• 
  pm run test\ passes with the new file

Labels

good first issue, help wanted