Style Web toasts to look more native on mobile

We intend to use the [Compound Web Toast component](https://compound.element.io/?path=/docs/compound-web_toast--docs) on mobile for Element Call. Some slight platform-dependent tweaks to the styles would help make the toasts look more native:

- They should have a dark background in dark themes and light background in light themes, rather than inverting the colors like the Web designs do.
- They have slightly different icon size, spacing, and padding values.

[Android designs](https://www.figma.com/design/G1xy0HDZKJf5TCRFmKb5d5/Compound-Android-Components?node-id=1997-45117&t=0VB10Cxrcsa2MkUn-4)

[iOS designs](https://www.figma.com/design/ux3tYoZV9WghC7hHT9Fhk0/Compound-iOS-Components?node-id=4467-88499&t=IMGQZl9gTV0AxEsu-4)

Recreating the iOS Liquid Glass effect in CSS is out of scope for this task. As [agreed with Aaron](https://www.figma.com/design/aOEkaJtaBmPy058V7uoqVr?node-id=1430-15300#1832970664), a plain, solid background on iOS is okay.

Start with the Compound Web Toast component documentation and compare its current styles with the linked Android and iOS designs. Update the mobile platform-dependent background, icon size, spacing, and padding styles; done means the web toasts match the platform designs while keeping the iOS background solid and excluding Liquid Glass.

Written by the indexing model from the issue text.