chore(app): shard the frontend unit tests because they take too long

[ddcc4]

Overview

The app frontend unit tests take > 30 minutes to run nowadays, which slows me down greatly when I'm waiting for the CI tests to finish. This change takes advantage of Vitest's sharding functionality (--shard) to split the tests up and run them in parallel on multiple GitHub runners.

The idea is that each shard runs a subset of the frontend unit tests. Each runner produces a Vitest result file (via --reporter=blob) and a coverage result file. We upload those files to temporary GitHub workflow artifact storage.

Then another runner waits for all the shards to finish, downloads the shard results, and merges them into a single Vitest result (with vitest --merge-reports), and merges the coverage results into a single file to send to Codecov.

This PR also fixes a small bug:

Previously, we did not specify our Codecov token in the Upload coverage report step, so it was failing with the error Please upload with the Codecov repository upload token to resolve issue, but no one had noticed the problem before.

Test Plan and Hands on Testing

Tested manually by carefully watching the CI runs on this PR.

Risk assessment

Low.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.12%. Comparing base (90de9fc) to head (c9b7398).

Additional details and impacted files

@@             Coverage Diff             @@
##             edge   #20341       +/-   ##
===========================================
+ Coverage   25.64%   57.12%   +31.47%     
===========================================
  Files        3639     3638        -1     
  Lines      302783   302719       -64     
  Branches    42297    42599      +302     
===========================================
+ Hits        77655   172918    +95263     
+ Misses     225104   129587    -95517     
- Partials       24      214      +190     

Flag	Coverage Δ
app	46.34% <ø> (+45.41%)	⬆️

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see 1842 files with indirect coverage changes

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