Fix panel header overflow on Rewards page

[zenparsing]

Resolves brave/brave-browser#28403

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run lint, npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

Before change

After change

[github-actions]

⚠️ PR head is an unsigned commit
commit: 321dc65d7f9ec6d8192008760c3ee4f56f113312
reason: unsigned
Please follow the handbook to configure commit signing
cc: @zenparsing

[github-actions]

⚠️ PR head is an unsigned commit
commit: 321dc65d7f9ec6d8192008760c3ee4f56f113312
reason: unsigned
Please follow the handbook to configure commit signing
cc: @zenparsing

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[bsclifton]

++

[Uni-verse]

Verified on Samsung Galaxy S21 5G using version:

Brave	1.50.28 Chromium: 110.0.5481.77 (Official Build) canary (32-bit) 
Revision	65ed616c6e8ee3fe0ad64fe83796c020644d42af-refs/branch-heads/5481@{#839}
OS	Android 13; Build/TP1A.220624.014

• Ensured that brave://rewards page displays correctly in non-US locale
• Ensured that header does not overflow on the page

Example	Example