fix(filters modal): layout cleanup, simplification, fixes

[rusackas]

SUMMARY

The new Add Filter/Divider buttons were getting hidden when the browser window was too short. "But it's flexbox, that shouldn't happen" I said. Then I started looking at all the onion layers of styled divs with random heights and overrides.

This PR cleans a bunch of junk up. There are fewer elements now, a few unnecessary overrides and styles are taken out, and things generally use more flexbox-native layouts. I could keep trimming until the cows come home, but I wanted to stop when the problem was fixed.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Video demo with narration:
https://github.com/user-attachments/assets/1ebaab79-71bc-40e5-975e-d09b77bf75e6

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[sadpandajoe]

/testenv up

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