Web backend font fixes

[mhsmith]

This fixes the issue mentioned at beeware/briefcase-web-static-template#16 (comment):

Style appears to have been lost from dialogs. The about dialog in Tutorial 0 displays in "browser default serif", not the Shoelace sans-serif default.

It looks like the style sheet simply never set a font on the top-level element, so the browser default was used on any elements which didn't specify their own font. This affected not only dialogs, but also Toga labels.

Also added a dummy Font implementation class, to fix the following error, which I guess may have been introduced by #2937:

  File "/lib/python3.12/site-packages/tutorial/app.py", line 9, in build
    box = toga.Box()
          ^^^^^^^^^^
  File "/lib/python3.12/site-packages/toga/widgets/box.py", line 25, in __init__
    super().__init__(id=id, style=style)
  File "/lib/python3.12/site-packages/toga/widgets/base.py", line 83, in __init__
    self.style.reapply()
  File "/lib/python3.12/site-packages/travertino/declaration.py", line 96, in reapply
    self.apply(style, getattr(self, style))
  File "/lib/python3.12/site-packages/toga/style/pack.py", line 131, in apply
    Font(
  File "/lib/python3.12/site-packages/toga/fonts.py", line 59, in __init__
    self._impl = self.factory.Font(self)
                 ^^^^^^^^^^^^^^^^^
  File "/lib/python3.12/site-packages/toga_web/factory.py", line 91, in __getattr__
    raise NotImplementedError(f"Toga's Web backend doesn't implement {name}")
NotImplementedError: Toga's Web backend doesn't implement Font

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[HalfWhitt]

Also added a dummy Font implementation class, to fix the following error, which I guess may have been introduced by #2937:

Ah, yeah that looks like it is indeed from #2942, because style.reapply() is actually getting called now. I encountered the same issue in Textual and fixed it the same way.

[freakboy3742]

👍