New CSS variable for mobile safe area insets

Shopify has introduced a new CSS custom property, `--shopify-safe-area-inset-bottom`, for embedded apps running on Shopify Mobile. Automatically set by App Bridge, it provides the pixel height of host UI overlays like the floating bottom navigation bar. Most apps require no changes since bottom padding is applied automatically, but apps with custom fixed-bottom elements (sticky footers, FABs) should use `var(--shopify-safe-area-inset-bottom, 0px)` in their CSS to avoid content being obscured. The change takes effect April 15th, 2026.