fix: DH-17454: Wrap Modal in SpectrumThemeProvider

[vbabich]

Wrap the Modal component attached to document.body in SpectrumThemeProvider.

Had to add zIndex: 0 and position: relative to the SpectrumThemeProvider to fix the issue with DatePicker popovers nested in the Modal rendering behind it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.67%. Comparing base (989b8ec) to head (bce2e25).
Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2169      +/-   ##
==========================================
+ Coverage   46.59%   46.67%   +0.07%     
==========================================
  Files         682      692      +10     
  Lines       38441    38628     +187     
  Branches     9578     9752     +174     
==========================================
+ Hits        17912    18028     +116     
- Misses      20519    20547      +28     
- Partials       10       53      +43     

Flag	Coverage Δ
unit	46.67% <ø> (+0.07%)	⬆️

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[dsmmcken]

seems like this would probably break something else, as that existing code was serving a purpose.

[vbabich]

I've reverted the change as it wasn't needed after wrapping the Modal in SpectrumThemeProvider.

[dsmmcken]

Confirmed working.

I checked whether we need z-index for popper so spectrum date picker works there too, but it actually doesn't work at all because the spectrum datepicker popper closes the our popper. That'll be a separate issue if we ever decide to place a picker in a poper. Probably not an issue for dh.ui as we wouldn't ever expose our popper, only the spectrum version.