AI Chat: small UI fixes

[nullhook]

Resolves brave/brave-browser#33732
Resolves brave/brave-browser#33944
Resolves brave/brave-browser#33943

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• 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:

[brave-builds]

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

[mkarolin]

strings++

[petemill]

why setting a fixed height on the menu to just the icon size? This seems strange when it's always going to need to be much taller

[nullhook]

leo-button-menu has an awkward height (1), so centering is off (2). therefore, i have added an explicit height (3)

[1]

[2]

[3]

[stephendonner]

Verification PASSED using

Brave | 1.62.8 Chromium: 119.0.6045.59 (Official Build) nightly (64-bit)
-- | --
Revision | 427a9c28d4368975377f8b6b503daee8a83684cd
OS | Windows 10 Version 22H2 (Build 19045.3636)

Shared Steps:

1. installed 1.62.8
2. launched Brave using --env-leo=staging --env-ai-chat.bsg=dev --env-ai-chat-premium.bsg=dev --enable-logging=stderr --v=1
3. purchased Leo Premium on account.bravesoftware.com
4. clicked on Leo in the sidebar to open Leo
5. clicked Accept and begin
6. loaded https://brave.com/leo-roadmap/

Issue 33732 - PASSED

(continued from Shared Steps)

7. hovered over the 3-dots ellipsis in the upper-right of the Leo pane

Confirmed the tooltip reads Leo settings

Issue 33944 - PASSED

Confirmed gray icons by default, with green icons once Premium-enabled

non-Premium - dark-gray icons	Premium - green lock icons

Issue 33943 - PASSED

Confirmed the 3-dots ellipsis is now vertically centered

fixed	before