refactor: Fix React Compiler hint in KeyboardShortcut

[frankieyan]

Part of #1066.

• File: src/components/keyboard-shortcut/keyboard-shortcut.tsx
• Violation: LogicalExpression cannot be safely reordered (BuildHIR::lowerReorderableExpression, Hint, line 122): the isMac default parameter value (navigator.platform?.toUpperCase().includes('MAC') ?? false).

Steps

1. Reproduce and confirm. Hint severity, but the component is still not memoized (confirmed in the compiled output).
2. Fix per #default-parameters-for-props: move the navigator.platform check out of the default-parameter position into the body so the expression is reorderable. Preserve the KeyboardShortcut.setTranslateKey public API.
3. Confirm the count dropped; let the hook update the record file.
4. Verify memoization in the compiled output.