Notifications: Fix filter tab color "leaking" to table view in dark scheme

[dvdchr]

Description

As titled, this fixes an issue where the background color for the filter tab in Notifications appears as if it's "leaking" to the table view side. See the below image for example:

The "leaked" space was an incidental bug from #16962, where we added some padding between the filter tab bar and the first table view header. It was a design decision to give some space between the two elements, so they don't feel too cramped visually — especially when the table header is already pretty compact.

Here's how it looks before and after the change in various configurations:

Scenarios	Before	After
iPhone, Dark
iPad, Dark
iPhone, Light
iPad, Light

To test

• Open either WordPress or Jetpack app.
• Switch to dark scheme.
• Go to the Notifications tab.
• 🔍 Verify that the filter tab bar's background color is no longer leaking.

Regression Notes

1. Potential unintended areas of impact
   Should be none.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manually tested the changes.

3. What automated tests I added (or what prevented me from doing so)
   N/A.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19772-1ab92df on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19772-1ab92df on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[alpavanoglu]

Issue is confirmed in trunk, resolved here ✅

[dvdchr]

Thanks @alpavanoglu!