feat(insights): Placeholders for Web Vitals boxes

[0Calories]

This improves the experience by adding placeholder cards for the web vitals while the data is loading. Previously, the vitals cards would be invisible and pop in after the data loaded, causing a very jarring layout shift.

Before

Kapture.2024-11-14.at.14.25.49.mp4

After

Kapture.2024-11-14.at.14.29.47.mp4

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ts/browser/webVitals/components/webVitalMeters.tsx	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #80769      +/-   ##
==========================================
+ Coverage   78.36%   78.41%   +0.04%     
==========================================
  Files        7211     7201      -10     
  Lines      319379   319094     -285     
  Branches    43986    43950      -36     
==========================================
- Hits       250297   250233      -64     
+ Misses      62703    62481     -222     
- Partials     6379     6380       +1