feat(frontend): add xs breakpoint (375px) to Tailwind @theme

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/layout.css

@@ -5,6 +5,9 @@
 
 /* ─── 2. Raw palette — never used directly in components ──────────────────── */
 @theme {
+	/* Breakpoints */
+	--breakpoint-xs: 375px;
+
 	/* Brand palette constants */
 	--palette-navy: #012851;
 	--palette-mint: #a1dcd8;