test(mobile): add missing SSR environment tests

[eunwoo-levi]

Overview

Problem

4 hooks in @react-simplikit/mobile were missing .ssr.test.ts files, which are required by the
project's contributing guidelines for all hooks that access browser APIs.

Solution

Add SSR environment tests (@vitest-environment node) for the 4 hooks, following the same pattern
as the existing SSR tests.

Hooks added in this PR

• useNetworkStatus
• usePageVisibility
• useAvoidKeyboard
• useKeyboardHeight

Additional Fix

• Found that @vitest/coverage-v8 was missing from @react-simplikit/mobile devDependencies,
  while the coverage config already existed in vitest.config.ts. Added the missing dependency.

Checklist

• Did you write the test code?
• Have you run yarn run fix to format and lint the code and docs?
• Have you run yarn run test:coverage to make sure there is no uncovered line?
• Did you write the JSDoc?