a11y: fix funny-toast color contrast using Bootstrap utility classes

[xpoes123]

Replaces the inline style="background-color: yellow" and text-danger on the #funny-toast header with Bootstrap utility classes that meet WCAG contrast requirements.

Problem

In both client/play/tossups/index.html and client/play/mp/room.html, the toast header used:

• style="background-color: yellow" — bypasses Bootstrap theming
• class="... text-danger" — red text on yellow background yields ~4.0:1 contrast, which fails WCAG AA for normal-weight text (threshold: 4.5:1)

Changes

• Removes inline style attribute; replaces with bg-warning Bootstrap utility class.
• Changes text-danger to text-dark, giving the bg-warning + text-dark pairing (~11.7:1 contrast) standard in Bootstrap's own documentation.

Risk & testing

Both files are updated identically. Toast IDs, role="alert", aria-live, and aria-atomic attributes are unchanged. The high-visibility yellow intent is preserved via bg-warning. node --check passes.