UX: mobile lightbox padding and styling improvements (#35494)

Adds a safe inset bottom for lightbox captions on mobile and removes
image padding on small screens. This change also applies bottom
depending on whether caption is being set or not.

How it looks:

<img width="225" height="487" alt="IMG_9334"
src="https://github.com/user-attachments/assets/d92d379a-ad63-44d0-8320-a0861b2a10e4"
/>

plugins/chat/assets/javascripts/discourse/components/chat-upload.gjs

@@ -86,7 +86,6 @@ export default class ChatUpload extends Component {
         style={{this.imageStyle}}
         loading="lazy"
         tabindex="0"
-        alt={{@upload.original_filename}}
         data-target-width={{this.size.width}}
         data-target-height={{this.size.height}}
         data-dominant-color={{@upload.dominant_color}}