style: Add per-tab-page class to LoadedTabPage for easier per-tab styling

[xitij2000]

Summary

Adds a dynamic CSS class to LoadedTabPage, so each course tab page (e.g. outline, progress, dates, discussion) gets a unique, predictable class name that can be targeted for tab-specific styling.

Description

Without the changes in this PR it difficult to apply styles scoped to a single tab (for example, adjusting heading colors only on the Progress tab) without resorting to fragile selectors based on content or position.

This change appends the active tab's slug as a class allowing theme authors to target specific tab pages.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.46%. Comparing base (8eb7cca) to head (a59b674).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1947   +/-   ##
=======================================
  Coverage   91.46%   91.46%           
=======================================
  Files         351      351           
  Lines        5820     5820           
  Branches     1390     1393    +3     
=======================================
  Hits         5323     5323           
  Misses        478      478           
  Partials       19       19           

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