Improves the error message when rendering text in WebGL mode without properly setting a font.

Problem:
The current error "only Opentype (.otf) and Truetype (.ttf) fonts are supported" is misleading when users have loaded a valid font but forgot to call textFont().

Solution:
Added clarification: "Make sure to set the font using textFont() before drawing text."

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated