Let the candidate resize the code editor font

[ColtenOuO]

Summary

The code editor font was fixed at 0.875rem. The only way to change it was browser zoom, which also scales the problem statement and the preflight overlay. This adds A- / A+ buttons to the editor toolbar that change only the editor and remember the choice between visits.

Close #218

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Summary by cubic

Lets candidates change the code editor font size with new A- / A+ buttons in the editor toolbar, instead of relying on browser zoom which also scales the problem statement and overlay. The choice is stored in localStorage and restored on the next visit.

• The textarea, highlight overlay, and gutter read a single custom property so they always stay aligned; the gutter width scales but never drops below its old minimum, so line numbers fit at every size.
• The buttons step through six fixed sizes, marked aria-disabled at the ends so keyboard focus stays on the button, and browser tests verify every size keeps the layers aligned.
• The default size is stored as no preference, so a later change of default reaches candidates who never moved off it.
• Closes Let the candidate change the code editor font size #218.

^{Written for commit 0ea0b65. Summary will update on new commits.}

[jserv]

Thank @ColtenOuO for contributing!