Fleet UI: Improve select targets dropdown

[RachelElysia]

Issue

Cerra #21276

Description

Select targets "searchy dropdown" improvements

• Dropdown loading state doesn't push UI at all
• Dropdown loading state, error state, and empty state all 225px (unless there is a search result, it's the height of the result)
• Loading spinner centered on dropdown
• Clicking out of results now closes the results except when loading (because results will reopen when loaded)

Screen recording of improvements

• Note: Video shows buggy results will reopen when loaded, hence fixed in PR
• https://www.loom.com/share/ec3f8e7b76f34ccfacca1cb4e9d74ba9

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
• Manual QA for all new/changed functionality

[codecov]

Codecov Report

Attention: Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.

Project coverage is 65.22%. Comparing base (dfc7289) to head (2d1fb93).
Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
...components/LiveQuery/TargetsInput/TargetsInput.tsx	72.22%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #22348      +/-   ##
==========================================
- Coverage   65.22%   65.22%   -0.01%     
==========================================
  Files        1496     1496              
  Lines      117247   117259      +12     
  Branches     3573     3516      -57     
==========================================
+ Hits        76477    76483       +6     
- Misses      33633    33640       +7     
+ Partials     7137     7136       -1     

Flag	Coverage Δ
frontend	52.38% <72.22%> (-0.01%)	⬇️

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[iansltx]

Got a few questions here for my understanding. Once I get those answered, LGTM. Keeping feedback as blocking since getting this in isn't urgent.