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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/deephaven/web-client-ui/pull/2169/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deephaven) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/deephaven/web-client-ui/pull/2169/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deephaven) | `46.67% <ø> (+0.07%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deephaven#carryforward-flags-in-the-pull-request-comment) to find out more.

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