Add Tests for useThemeInit Hook

[Mosas2000]

Description

The theme initialization hook has no tests. frontend/src/hooks/useThemeInit.ts has no corresponding test file, so theme initialization and persistence logic is untested.

Requirements and context

• Add useThemeInit.test.tsx
• Cover theme loading, applying, and persistence
• Use existing hook test conventions
• Ensure tests pass under the frontend vitest config

Suggested execution

Fork the repo and create a branch

git checkout -b test/hooks-use-theme-init

Implement changes

• Add the hook test file
• Cover happy paths and edge cases
• Verify with npm run test --workspace=frontend

Example commit message

test: add tests for useThemeInit hook

Guidelines

• Assignment required before starting
• PR must include a closing reference to this issue
• Timeframe: 72 hours