fix(ui): restore grouped textarea top padding (#50843)

## Problem

- Grouped textareas render text too close to the top border. The
standalone `Textarea` retains its normal top padding.
- `InputGroupTextarea` has overridden the inner textarea with `py-0`
since #44282 on 31 March 2026. The group initially supplied `py-2`;
#44387 removed that outer padding later the same day, leaving no top
spacing. The override was retained during the border and surface updates
in #44690 and #48887.
- The textarea component page had no grouped textarea example, so this
difference was easy to miss.

## Solution

- Give the grouped textarea `pt-2` while retaining `pb-0` for block-end
addons. The outer group continues to own its border and focus styles.
- Add a “With addon” example to the textarea component page to keep this
composition visible.

## Review instructions

- Open `/design-system/docs/components/textarea` and find “With addon”.
Confirm its text has the same top inset as the default textarea and the
block-end addon remains below the input.
- Open `/design-system/docs/ui-patterns/forms` and find “Textarea
Multi-line text input for longer content with addon”. Confirm the text
has the same top inset as the standalone textarea example immediately
above it.

## Checklist

- [x] I have read
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added a textarea example with a supporting addon below the text field,
showing a 120-character limit.
* **Bug Fixes**
* Clicking a block-end addon now focuses its textarea as well as an
input.
* **Documentation**
* Added the textarea-with-addon example to the textarea component
documentation.
* **Style**
* Added top padding to textareas within input groups while keeping
bottom padding unchanged, adjusting spacing above entered text.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/design-system/content/docs/components/textarea.mdx

@@ -72,6 +72,12 @@ import { Textarea } from '@/components/ui/textarea'
 
 <ComponentPreview name="textarea-with-button" />
 
+### With addon
+
+An addon places supporting content inside the same bordered field as the textarea. Wrap `InputGroupTextarea` and `InputGroupAddon` in `InputGroup`, and use `align="block-end"` to place the addon below the text.
+
+<ComponentPreview name="textarea-with-addon" />
+
 ### Form
 
 <ComponentPreview name="textarea-form" />

apps/design-system/registry/default/example/textarea-with-addon.tsx

@@ -0,0 +1,12 @@
+import { InputGroup, InputGroupAddon, InputGroupText, InputGroupTextarea } from 'ui'
+
+export default function TextareaWithAddon() {
+  return (
+    <InputGroup className="w-full max-w-sm">
+      <InputGroupTextarea placeholder="Type your message here." rows={4} maxLength={120} />
+      <InputGroupAddon align="block-end">
+        <InputGroupText>120 character limit</InputGroupText>
+      </InputGroupAddon>
+    </InputGroup>
+  )
+}

apps/design-system/registry/examples.ts

@@ -1021,6 +1021,12 @@ export const examples: Registry = [
     registryDependencies: ['textarea', 'form'],
     files: ['example/textarea-form.tsx'],
   },
+  {
+    name: 'textarea-with-addon',
+    type: 'components:example',
+    registryDependencies: ['textarea', 'input-group'],
+    files: ['example/textarea-with-addon.tsx'],
+  },
   {
     name: 'textarea-with-button',
     type: 'components:example',