test: cover ThemeContext system-preference change listener

[Truphile]

I have successfully completed the tasks to add tests for the OS-preference change listener in ThemeContext.tsx .

Here's a summary of the actions taken:

1. Created a new branch test/theme-context-system-listener as requested.
2. Fixed a pre-existing testing bug: The current test suite had jest.fn() accidentally used inside
   src/context/tests/ThemeContext.test.tsx , which was causing vitest to fail. I replaced these occurrences
   with vi.fn() .
3. Implemented missing tests: The existing tests already covered most of the requirements (such as following OS
   changes, stopping the listener when manually switched, and resuming when system preference is reselected). I
   extended the test suite by adding an unmount test to verify that the removeEventListener is correctly called,
   ensuring no memory leaks or unexpected updates occur.
4. Verified coverage and correctness: Ran the tests for ThemeContext.test.tsx using Vitest to ensure all tests
   pass cleanly.
5. Committed changes: Pushed the commit with the message test: cover ThemeContext system-preference change
   listener per your instructions.

closes #328

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