Replace fa-spinner with rounded SVG ring spinner

Webtrees 2.2.6 introduced .wt-icon-spinner wrapping a FontAwesome
fa-spinner. Replaces it (and the existing .wt-ajax-load Bootstrap
spinner-border) with a single rounded-cap SVG arc tinted via
--bs-primary. Hides FA's JS-injected <svg> child so only the new
spinner renders.

src/scss/theme.scss

@@ -533,14 +533,49 @@ select {
   }
 }
 
+@keyframes wt-spin {
+  to { transform: rotate(360deg); }
+}
+
+$wt-spinner-svg: "data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 48 48'%3E%3Ccircle cx='24' cy='24' r='19' fill='none' stroke='black' stroke-width='4' stroke-linecap='round' stroke-dasharray='89.54 29.85'/%3E%3C/svg%3E";
+
+.wt-icon-spinner {
+  display: block;
+  width: 3rem;
+  height: 3rem;
+  margin-inline: auto;
+  color: var(--bs-primary);
+
+  > * { display: none !important; }
+
+  &::before {
+    content: '';
+    display: block;
+    width: 100%;
+    height: 100%;
+    background-color: currentColor;
+    mask: url($wt-spinner-svg) center / contain no-repeat;
+    -webkit-mask: url($wt-spinner-svg) center / contain no-repeat;
+    animation: wt-spin 0.9s linear infinite;
+  }
+}
+
 [dir] .wt-ajax-load:empty {
-  height: 2rem;
+  height: 3rem;
+  position: relative;
 
   &::before {
     content: '';
     position: absolute;
     left: 50%;
-    @extend .spinner-border;
+    top: 0;
+    width: 3rem;
+    height: 3rem;
+    margin-left: -1.5rem;
+    background-color: var(--bs-primary);
+    mask: url($wt-spinner-svg) center / contain no-repeat;
+    -webkit-mask: url($wt-spinner-svg) center / contain no-repeat;
+    animation: wt-spin 0.9s linear infinite;
   }
 }