♿️ Fix AA contrast and move mobile language selector to header row

Darken burufugu primary-600 to pass 4.5:1 on white, bump the search
shortcut hint to neutral-500, and surface the language switcher in
the visible mobile header row instead of the menu dialog.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/css/schemes/burufugu.css

@@ -19,7 +19,7 @@
   --color-primary-300: 138, 197, 250;
   --color-primary-400: 96, 172, 246;
   --color-primary-500: 74, 158, 240;
-  --color-primary-600: 52, 126, 219;
+  --color-primary-600: 47, 115, 205;
   --color-primary-700: 42, 98, 189;
   --color-primary-800: 38, 76, 152;
   --color-primary-900: 30, 58, 110;