Tweak and fix toggle checkboxes (#30527)

Fixes: https://github.com/go-gitea/gitea/issues/30524. Slightly restyled
them so that the "knob" is contained inside the background.

<img width="179" alt="Screenshot 2024-04-16 at 21 58 09"
src="https://github.com/go-gitea/gitea/assets/115237/be94517b-9cb7-46e2-ae96-fcf6767ce4ba">
<img width="187" alt="Screenshot 2024-04-16 at 21 58 50"
src="https://github.com/go-gitea/gitea/assets/115237/c13a1959-5c5a-4e88-9225-e5f6fb72e3e0">

(cherry picked from commit 5ccd042f7080e1f4ef4b96591e1b1002a4826115)

web_src/css/modules/checkbox.css

···
 }
 .ui.toggle.checkbox input {
   width: 3.5rem;
-  height: 1.5rem;
+  height: 21px;
   opacity: 0;
   z-index: 3;
 }
···
   content: "";
   z-index: 1;
   top: 0;
-  width: 3.5rem;
-  height: 1.5rem;
+  width: 49px;
+  height: 21px;
   border-radius: 500rem;
   left: 0;
 }
 .ui.toggle.checkbox label::after {
   background: var(--color-white);
+  box-shadow: 1px 1px 4px 1px var(--color-shadow);
   position: absolute;
   content: "";
   opacity: 1;
   z-index: 2;
-  width: 1.5rem;
-  height: 1.5rem;
-  top: 0;
-  left: 0;
+  width: 18px;
+  height: 18px;
+  top: 1.5px;
+  left: 1.5px;
   border-radius: 500rem;
   transition: background 0.3s ease, left 0.3s ease;
 }
 .ui.toggle.checkbox input ~ label::after {
-  left: -0.05rem;
+  left: 1.5px;
 }
 .ui.toggle.checkbox input:checked ~ label::after {
-  left: 2.15rem;
+  left: 29px;
 }
 .ui.toggle.checkbox input:focus ~ label::before,
 .ui.toggle.checkbox label::before {