Add user preferences for theme and font size.

Persist per-account UI settings so the dashboard can restore default light/dark mode and font size on login.

components/users/userModel.js

@@ -114,6 +114,18 @@ const userSchema = new mongoose.Schema({
   adminNotes: {
     type: [String],
     default: []
+  },
+  preferences: {
+    theme: {
+      type: String,
+      enum: ['light', 'dark', 'system'],
+      default: 'system'
+    },
+    fontSize: {
+      type: String,
+      enum: ['small', 'medium', 'large'],
+      default: 'medium'
+    }
   }
 }, {
   timestamps: true