Reorganize UI controls and improve form interactions (#500)

Reorganizes control placement in the playground interfaces and
improves form interactions for better UX, particularly on mobile
devices.

## Key Changes

- **AudioInterface & ImageInterface**: Moved "Clear" buttons from the
top control bar into the action button group below the form inputs for
better visual hierarchy and logical grouping
- **ImageInterface**: 
- Added prompt clearing to the `clearImage()` function so the input
field is reset when clearing generated images
- Updated Clear button disabled state to also check if prompt is empty,
allowing users to clear an empty prompt
- Added responsive flex styling (`flex-1 md:flex-none`) to the Clear
button for better mobile layout
- **ExpandableTextarea**: 
- Imported `untrack` from Svelte to properly handle reactive
dependencies
- Wrapped `expandedValue.length` in `untrack()` to prevent unnecessary
reactivity when setting cursor position
- Improved button visibility on mobile by changing opacity from
`opacity-0` to `opacity-60` with `md:opacity-0` breakpoint, making the
expand button more discoverable on touch devices

## Implementation Details
The `untrack()` usage in ExpandableTextarea ensures that reading the
text length doesn't create a reactive dependency, preventing potential
infinite loops while still allowing the effect to run when `isExpanded`
changes.

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