Fix GitHub link unconditionally navigating to Discord

The GitHub link in the site header across all pages is unconditionally
linking to Discord, not GitHub. More details in the issue, but the gist
is it appears to be a regression from a large refactor merged 2 days
ago: fef6139

All the context was already wired up, it just appears to be using the
wrong context variable. An easy fix! Hopefully.

Tested locally by smoke testing the link across a few pages and project
roots.

Closes #1482

Author: PsychoLlama

osmium/src/ui/layout/main-header.tsx

@@ -113,7 +113,7 @@ export function MainHeader(_props: MainHeaderProps) {
 				<div class="order- flex basis-0 items-center justify-end gap-4 lg:order-2">
 					<ClientSearch />
 					<a
-						href={`${config().themeConfig?.discord}/${project().projects[project().current].path || "solid"}`}
+						href={`${config().themeConfig?.github}/${project().projects[project().current].path || "solid"}`}
 						class="group"
 						aria-label="GitHub"
 						target="_blank"