feat: add gradient overlay to price widget appbar

[jvsena42]

Related to #169
This PR adds a gradient fade effect below the AppTopBar in the Price widget edit screen, allowing scrollable content to smoothly fade as it scrolls behind the header.

Description

1. Wraps the AppTopBar and scrollable content in a Box for z-layer ordering
2. Positions the scrollable Column behind the AppTopBar overlay
3. Adds a vertical gradient from the background color to transparent below the AppTopBar
4. Adjusts content top spacing to account for the AppTopBar and gradient height
5. Fixes the vertical padding between the the items

Preview

tobpar-gradient-bg.webm

QA Notes

1. Navigate to Settings > Widgets > Price > Edit
2. Verify the gradient fade effect appears below the AppTopBar
3. Scroll the content and confirm:
   • Content smoothly fades as it passes behind the AppTopBar
   • AppTopBar remains fully visible and interactive
   • Back button and drawer icon work correctly

[ovitrif]

utAck