fonts: consolidate font-family to global stack, self-host subset emoji font

Remove redundant `font-family: sans-serif` from 18 component-level
declarations — they now inherit the Helvetica/Arial/Noto Sans stack
from the global `*` selector in app.css.

Add self-hosted NotoColorEmoji subset (56 KB, down from 11 MB) as
fallback for systems without the Noto Color Emoji font installed.
The subset is generated at prebuild time via pyftsubset with a fixed
list of the ~32 emojis actually used on the site.

src/lib/components/Header.svelte

@@ -116,7 +116,6 @@ nav[hidden]{
 :global(.site_header li>a)
 {
 	text-decoration: none;
-	font-family: sans-serif;
 	font-size: 1.2rem;
 	color: inherit;
 	border-radius: var(--radius-pill);
@@ -127,7 +126,6 @@ nav[hidden]{
 :global(a.entry:visited)
 {
 	text-decoration: none;
-	font-family: sans-serif;
 	font-size: 1.2rem;
 	color: white !important;
 	border-radius: var(--radius-pill);