Two fonts are drawn into the share card image (src/card.ts), so they ship with the Worker:

• JetBrainsMono-Bold.ttf: JetBrains Mono 700, Latin subset. License: OFL-jetbrains-mono.txt.
• IBMPlexSans-Medium.ttf: IBM Plex Sans 500, Latin subset. License: OFL-ibm-plex-sans.txt.

Both are the Latin subsets published by Fontsource. The layout relies on JetBrains Mono being monospaced (every glyph is 0.6 em wide), so swapping that one for a proportional font needs changes in src/card.ts.