test: cover desktop runtime detection

[McanKul]

Goal

Add deterministic unit coverage for isTauriRuntime() in src/lib/tauriEnv.ts, which separates desktop-only behavior from the browser preview.

Scope

• Add src/lib/tauriEnv.test.ts.
• Test the helper by installing and restoring controlled window markers.
• Keep the tests independent of a running Tauri application.

Acceptance criteria

• Returns false when window is unavailable.
• Returns false when a browser-like window has no Tauri markers.
• Returns true for each supported marker independently: __TAURI_INTERNALS__, __TAURI__ and isTauri.
• Falsey marker values do not produce a false positive.
• Every test restores global state so test order cannot affect the result.
• Production behavior stays unchanged unless a test exposes a documented defect.

Verification

npm test -- src/lib/tauriEnv.test.ts
npm run typecheck

[McanKul]

Resolved by #58 and merged into development. Thanks for adding the focused runtime coverage.