feat(progress): allow customizing completion percentage precision

[Agrendalath]

The completion donut chart on the progress tab always rounds its percentages to whole numbers. In courses with a large number of units, a learner can complete several units in a row without the chart moving at all, which is confusing.

This PR adds a COMPLETION_PERCENTAGE_PRECISION environment variable that controls how many decimal places the chart's percentages are rounded to. It defaults to 0, so it is fully backward -compatible.

Testing instructions

1. Set the COMPLETION_PERCENTAGE_PRECISION in the MFE envs (e.g., via the MFE_CONFIG) and visit the course progress page.

Deadline

"None"

Other information

Private-ref: BB-11198

TODO: The only missing thing is the product proposal.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.60%. Comparing base (dcc62ac) to head (3d7ee61).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2053      +/-   ##
==========================================
+ Coverage   93.58%   93.60%   +0.01%     
==========================================
  Files         367      367              
  Lines        6004     6007       +3     
  Branches     1416     1417       +1     
==========================================
+ Hits         5619     5623       +4     
+ Misses        368      367       -1     
  Partials       17       17              

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