fix(styles): enforce consistent font-family on code blocks

[RUKAYAT-CODER]

closes #190

Summary

Fixes inconsistent ont-family in and

 elements by adding a global CSS rule that enforces the design-system monospace font (--font-mono / Geist Mono) across all code blocks.

Changes

src/app/globals.css: added code, pre, kbd, samp rule using �ar(--font-mono, ...) fallback chain; added pre { overflow-x: auto } to prevent layout breakage on long lines
src/tests/design-system/code-block-font-family.test.tsx: added invariant tests verifying ont-mono class usage on code and pre elements