style: bake in 90% zoom scale via root font-size

Set html { font-size: 90% } so the whole UI (sidebar, typography, spacing)
renders at the ~90% browser-zoom look the owner prefers, consistently on
both the Pi and the remote server.

static/yaris.css

@@ -1,5 +1,12 @@
 /* ── Layout ──────────────────────────────────────────────────────────────── */
 
+/* The UI is designed to look best at ~90% browser zoom, so bake that scale in
+   by shrinking the root font size — every rem-based size (sidebar, typography,
+   spacing) follows. */
+html {
+    font-size: 90%;
+}
+
 :root {
     --sidebar-width: 14rem;
     --sidebar-collapsed-width: 4rem;