fix(web): font size was not consistently set

Fixes #5779.
Fixes #5731 (I believe mitigation is sufficient to close this issue).

A variety of interrelated font and font size display issues resolved:
1. KVK font was not applied early enough for size calculations, which
   meant that we were calculating font scaling per key based on a
   default font when transforming from the KVK data for desktop devices
   (defaultLayout.ts)
2. Font scaling for non-default layers was calculated when elements were
   not visible and had no size information, giving incorrect values. To
   resolve this, font scaling is now calculated when a layer is made
   visible, which had performance impacts; resolved by reducing
   unnecessary `layer.refreshLayout()` calls; see performance point 1
   below (oskView.ts:layerChangeHandler())
3. `getViewportScale()` would return an incorrect scaled value when
   emulating touch devices on a desktop browser (kmwutils.ts)
4. After switching keyboards, the device-specific scaling factor was not
   maintained (oskView.ts:refreshLayout())

Related performance improvements:
1. Multiple calls to `layer.refreshLayout()` in `refreshLayout()` have
   been eliminated, and only the currently visible layer is now
   refreshed. This dramatically reduces the number of calls to
   `getIdealFontSize()` which was the primary concern of #5731.
   (visualKeyboards.ts)
2. Unnecessary use of `innerHTML` replaced with `innerText`
   (oskBaseKey.ts)

Minor Keyman Developer performance improvement:
1. The web debugger no longer recalculates the OSK twice (test.js)