fix(checkbox): align checkbox properly in item using start alignment (#29850)

Issue number: resolves #29837

---------

## What is the current behavior?
The checkbox is not aligned properly to the top when using a long label
with `alignment="start"` inside of an `ion-item`:

```html
<ion-item>
  <ion-checkbox justify="start" alignment="start">
    <ion-label class="ion-text-wrap">
      Enable Notifications Enable Notifications Enable Notifications
    </ion-label>
  </ion-checkbox>
</ion-item>
```

## What is the new behavior?
- Applies the same margin to the `.native-wrapper` (checkbox) as the
label.
- Adds a screenshot test to verify the alignment

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

## Other information

| Before | After |
| --- | --- |
|
![before](https://github.com/user-attachments/assets/53579523-e8b5-4152-ae91-14847cb395e4)
|
![after](https://github.com/user-attachments/assets/0d7315ed-3294-4a27-82fe-6900eb9db1c0)
|
|
![before](https://github.com/user-attachments/assets/ca025a94-4ef7-44b4-85d0-5183e4326814)
|
![after](https://github.com/user-attachments/assets/fab60703-1196-48e8-a485-2f33c4893aba)
|

- [Label
Preview](https://ionic-framework-git-rou-11163-ionic1.vercel.app/src/components/checkbox/test/label)
- [Item
Preview](https://ionic-framework-git-rou-11163-ionic1.vercel.app/src/components/checkbox/test/item)

> [!NOTE]
> The alignment on the Material Design checkbox is still slightly off. I
could add margin directly to its checkbox but then it would change the
margin of the checkbox in all use cases.

Author: brandyscarney

core/src/components/checkbox/checkbox.scss

@@ -87,7 +87,10 @@
   overflow: hidden;
 }
 
-:host(.in-item) .label-text-wrapper {
+// Checkboxes that are not slotted inside an item and are not used with a
+// stacked label should have margins equal to those of the label.
+:host(.in-item) .label-text-wrapper,
+:host(.in-item:not(.checkbox-label-placement-stacked):not([slot])) .native-wrapper {
   @include margin($checkbox-item-label-margin-top, null, $checkbox-item-label-margin-bottom, null);
 }
 

core/src/components/checkbox/test/item/checkbox.e2e.ts

@@ -51,7 +51,7 @@ configs({ directions: ['ltr'] }).forEach(({ title, screenshot, config }) => {
   });
 });
 
-configs({ directions: ['ltr'], modes: ['md'] }).forEach(({ title, screenshot, config }) => {
+configs({ directions: ['ltr'] }).forEach(({ title, screenshot, config }) => {
   test.describe(title('checkbox: long label in item'), () => {
     test('should render margins correctly when using long label in item', async ({ page }) => {
       await page.setContent(
@@ -69,6 +69,28 @@ configs({ directions: ['ltr'], modes: ['md'] }).forEach(({ title, screenshot, co
       const list = page.locator('ion-list');
       await expect(list).toHaveScreenshot(screenshot(`checkbox-long-label-in-item`));
     });
+    test('should render margins correctly when using long label in item with start alignment', async ({
+      page,
+    }, testInfo) => {
+      testInfo.annotations.push({
+        type: 'issue',
+        description: 'https://github.com/ionic-team/ionic-framework/issues/29837',
+      });
+      await page.setContent(
+        `
+          <ion-list>
+            <ion-item>
+              <ion-checkbox justify="start" alignment="start">
+                <ion-label class="ion-text-wrap">Enable Notifications Enable Notifications Enable Notifications</ion-label>
+              </ion-checkbox>
+            </ion-item>
+          </ion-list>
+        `,
+        config
+      );
+      const list = page.locator('ion-list');
+      await expect(list).toHaveScreenshot(screenshot(`checkbox-long-label-in-item-align-start`));
+    });
   });
 
   test.describe(title('checkbox: stacked label in item'), () => {