Improve text contrast in programming exercise submission

[LeoDanielA01]

Two small visual fixes on the Programming Exercise Submission page.

1. Run button : removed a hardcoded gray text color that was
overriding the button's default white label.

2. Compiler error message : the error text was too light to read
comfortably. Increased the red shade so it stands out more clearly
when something goes wrong.

Before:

After:

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

The changes use established component defaults and an existing red text utility to improve contrast without altering behavior.

_{Reviews (1): Last reviewed commit: "fix: improve text contrast in programmin..." | Re-trigger Greptile}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.10%. Comparing base (13d2114) to head (da4a28d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2698   +/-   ##
========================================
  Coverage    77.09%   77.10%           
========================================
  Files          189      189           
  Lines        12146    12146           
========================================
+ Hits          9364     9365    +1     
+ Misses        2782     2781    -1     

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