feat(ui): change red text when instructor not found

[aaronthechen]

Fixes #404.

Updated text when an instructor is not found, changed text size to small, and removed italics.

Before:
"Instructor-specific data is not available for this course, showing course-wide data instead"
After:
"We couldn't find instructor-specific grades, so here are the grades for all (insert course number) sections."

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This change is 

[doprz]

I recommend moving the regex logic for formatting the semester value outside of the TSX block and into a computed variable or React.useMemo() (next to the semester variable would be ideal). While the performance improvement might not be significant in most cases, this change enhances code clarity and maintainability. By preprocessing the value once, we avoid unnecessary recalculations on every render, especially if the component re-renders for reasons unrelated to the semester. This approach also makes the TSX cleaner, separating logic from presentation, which improves readability. Although it's a nit, adopting this pattern can make the code easier to extend in the future and reduce potential mistakes when modifying the formatting logic.

[IsaDavRod]

LGTM