Fix Apple modifier fallback for unavailable platform hints

[czarandy]

When client hints report a missing, blank, or Unknown platform on an Apple device, Kbd now displays Command and useHotkey matches Command shortcuts by falling back to navigator.platform. Nonempty, known hints continue to take precedence, including non-Apple hints.

Adds regression coverage for both consumers, accessible Kbd labels, exact hotkey modifiers, malformed hints, Apple device fallbacks, and environments without a navigator. Existing platform-adaptive Kbd stories cover the unchanged UI.

Closes #643.

Validation:

• All 103 targeted tests passed; the new tests reproduced 18 failures before the fix.
• Changed-file ESLint, TypeScript, formatting, library build, and package consumer smoke test passed.
• Full suite hit unrelated timeouts under contention. Stopped remaining local validation at the user's request to release RAM; full-suite, repository lint, and site-build results are left to CI.

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Project	Deployment	Actions	Updated
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