Fix unclickable checkboxes (go-gitea#30195)

Fix go-gitea#30185, regression from
go-gitea#30162.

The checkboxes were unclickable because the label was positioned over
the checkbox with `padding`. Now it uses `margin` so the checkbox itself
will be clickable in all cases.

Secondly, I changed the for/id linking to also add missing `for`
attributes when `id` is present. The other way around (only `for`
present) is currently not handled and I think there are likey no
occurences in the code and introducing new non-generated `id`s might
cause problems elsewhere if we do, so I skipped on that.

web_src/css/modules/checkbox.css

@@ -41,7 +41,7 @@ input[type="radio"] {
 
 .ui.checkbox label,
 .ui.radio.checkbox label {
-  padding-left: 1.85714em;
+  margin-left: 1.85714em;
 }
 
 .ui.checkbox + label {

web_src/js/modules/fomantic/checkbox.js

@@ -6,10 +6,19 @@ export function initAriaCheckboxPatch() {
     if (el.hasAttribute('data-checkbox-patched')) continue;
     const label = el.querySelector('label');
     const input = el.querySelector('input');
-    if (!label || !input || input.getAttribute('id') || label.getAttribute('for')) continue;
-    const id = generateAriaId();
-    input.setAttribute('id', id);
-    label.setAttribute('for', id);
+    if (!label || !input) continue;
+    const inputId = input.getAttribute('id');
+    const labelFor = label.getAttribute('for');
+
+    if (inputId && !labelFor) { // missing "for"
+      label.setAttribute('for', inputId);
+    } else if (!inputId && !labelFor) { // missing both "id" and "for"
+      const id = generateAriaId();
+      input.setAttribute('id', id);
+      label.setAttribute('for', id);
+    } else {
+      continue;
+    }
     el.setAttribute('data-checkbox-patched', 'true');
   }
 }