fix(theme): default to dark, polish theme cycle visibility and hover

New installs land on plain dark instead of ultra-dark. The cycle button
icon now has an explicit colour so it stays visible inside the mobile
drawer (the previous color:inherit didn't cascade through the teleported
node), and hover/focus matches the menu's blue across sidebar, login,
and sub pages.

frontend/src/composables/useTheme.js

@@ -16,7 +16,7 @@ function readBool(key, fallback) {
 }
 
 const isDark = readBool(STORAGE_DARK, true);
-const isUltra = readBool(STORAGE_ULTRA, true);
+const isUltra = readBool(STORAGE_ULTRA, false);
 
 export const theme = reactive({
   isDark,