style(splash): set Athelas as primary font and italicize button text

- Prioritize Athelas in --font-system font stack with system fonts as fallback
- Apply italic style to splash button heading and label for typography emphasis

src/app/splash/components/splash-button.module.css

@@ -12,6 +12,7 @@
   font-family: var(--font-athelas);
   font-size: var(--font-size-xxl);
   font-weight: 700;
+  font-style: italic;
   color: #ffffff;
   background: transparent;
   border: 0;
@@ -43,6 +44,7 @@
   color: #ffffff;
   font-weight: 700;
   cursor: pointer;
+  font-style: italic;
   display: inline-flex;
   align-items: center;
   justify-content: center;