refactor: merge Language and Theme settings into one two-column card

Language and Theme were two separate Cards stacked on top of each other —
both are pure client-side display preferences, so splitting them side by
side (language left, theme right) saves a card's worth of vertical space.
Collapses back to a single column on mobile.

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "aurum-frontend",
   "private": true,
-  "version": "0.19.0",
+  "version": "0.19.1",
   "type": "module",
   "scripts": {
     "dev": "vite",