fix: add unit to --y CSS variable to fix vertical drag

`--y` is initialized as `0` (unitless), which works in bare
`translate3d(0, var(--y), 0)` contexts. However, when
`draggableDirection` is `'y'`, the drag handler builds
`calc(${delta}px + var(--y))` — and `calc()` requires compatible
units on both operands. `calc(50px + 0)` is invalid CSS, while
`calc(50px + 0px)` is valid.

This causes vertical drag to have no visible effect — the toast
fades via opacity but doesn't move, because the transform is
silently invalid.

Changing `--y: 0` to `--y: 0px` fixes vertical dragging with no
impact on existing behavior, since `0` and `0px` are identical
in all non-calc contexts.

Author: janpaepke

src/style.css

@@ -91,7 +91,7 @@
 }
 
 .Toastify__toast {
-  --y: 0;
+  --y: 0px;
   position: relative;
   touch-action: none;
   width: var(--toastify-toast-width);