docs: apply prefer/avoid blocks in UI with comments

Apply inline comments with proper Prefer and Avoid UI blocks so
guidance is more visible and consistent across the documentation.

Author: MeAkib

adev/src/content/guide/forms/signals/custom-controls.md

@@ -416,15 +416,17 @@ When you add `min()` and `max()` validation rules to the schema, the FormField d
 
 IMPORTANT: Don't implement validation logic in your control. Define validation rules in the form schema and let your control display the results:
 
-```typescript
+```ts {avoid}
 // Avoid: Validation in control
 export class BadControl implements FormValueControl<string> {
   value = model<string>('');
   isValid() {
     return this.value().length >= 8;
   } // Don't do this!
 }
+```
 
+```ts {prefer}
 // Good: Validation in schema, control displays results
 accountForm = form(this.accountModel, (schemaPath) => {
   minLength(schemaPath.password, 8, {message: 'Password must be at least 8 characters'});

adev/src/content/guide/forms/signals/models.md

@@ -74,14 +74,16 @@ const usernameField = loginForm.username;
 
 Form models should provide initial values for all fields you want to include in the field tree.
 
-```ts
+```ts {prefer}
 // Good: All fields initialized
 const userModel = signal({
   name: '',
   email: '',
   age: 0,
 });
+```
 
+```ts {avoid}
 // Avoid: Missing initial value
 const userModel = signal({
   name: '',