ui: Handle null navigator.keyboard

[iw4p]

Problem

In some browsers, the keyboard property exists on navigator but its value is null.

The existing property-presence check succeeds and then calls getLayoutMap() on null, crashing the Perfetto home page:

TypeError: Cannot read properties of null (reading 'getLayoutMap')

Observed with Perfetto UI v58.2 on Chrome 150 on macOS.

Fix

Declare navigator.keyboard as nullable and check its value before calling getLayoutMap().

Unsupported environments now throw the existing NotSupportedError, allowing the UI to use its fallback keyboard layout.

I solved it with declare global because I saw they've been used in other parts, could be fixed with cast but ugly, or runtime guard which's verbose..

Additionally getLayoutMap() genuinely returns a Promise according to the Keyboard Map specification, this is the reason I changed it.

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[stevegolton]

Thanks for the patch!