fix: clean up theme matchMedia listener to prevent leak

[ozantaybas]

What

The change listener on the matchMedia("(prefers-color-scheme: dark)")
query in themeStore.ts was added but never removed, leaking the listener.

How

Extracted the handler and the MediaQueryList into named references, and
added an exported cleanupThemeListener() to allow proper removal.
Behavior is unchanged.

Testing

Added a unit test verifying cleanupThemeListener() calls
removeEventListener with the same handler. All themeStore tests pass
(11/11) and type-check is clean.

Closes #117

[ozantaybas]

Heads up: the failing Frontend CI check is unrelated to this PR.
All 10 failures come from src/components/editor/QueryToolbar.tsx:346
(Cannot destructure property 'limitEnabled' of 'useSettingsStore(...)'),
which this PR doesn't touch — it only changes themeStore.ts and its test.
The themeStore tests pass (11/11) locally and type-check is clean.
The Rust checks also appear unrelated to a frontend-only change.
Happy to rebase if you'd like once that unrelated test is fixed.