fix(zone-modal): center slider thumb on track, drop dead code, cover a11y

- Center webkit slider thumb on 4px track (margin-top: -4.5px) so the
  opacity bullet no longer sits below the bar.
- Drop redundant inline accentColor; CSS module owns track color.
- Remove dead ::-ms-* prefix rules.
- Tests: assert aria-labels on grid/select triggers, cover getFontLabel
  happy path + unknown-value fallback via the Font selector DOM.

frontend/src/components/modals/GroupRectModal.module.css

@@ -1,4 +1,15 @@
-/* Custom colored slider track for accent color */
+/* Slider container: strip native chrome so custom track/thumb align cleanly */
+.slider-thumb {
+  -webkit-appearance: none;
+  appearance: none;
+  background: transparent;
+  height: 13px; /* match thumb height so vertical centering is the input's box center */
+}
+.slider-thumb:focus {
+  outline: none;
+}
+
+/* Track */
 .slider-accent::-webkit-slider-runnable-track {
   height: 4px;
   background: #00d4ff;
@@ -9,23 +20,17 @@
   background: #00d4ff;
   border-radius: 2px;
 }
-.slider-accent::-ms-fill-lower,
-.slider-accent::-ms-fill-upper {
-  background: #00d4ff;
-  border-radius: 2px;
-}
 
-/* Ensure the slider bar is visible on dark backgrounds */
-.slider-thumb {
-  background: transparent;
-}
-/* Custom slider thumb for smaller circle */
+/* Thumb — must offset on webkit so it centers on the 4px track */
 .slider-thumb::-webkit-slider-thumb {
+  -webkit-appearance: none;
+  appearance: none;
   width: 13px;
   height: 13px;
   border-radius: 50%;
   background: #00d4ff;
   border: 2px solid #21262d;
+  margin-top: -4.5px; /* (13 - 4) / 2 */
 }
 .slider-thumb::-moz-range-thumb {
   width: 13px;
@@ -34,10 +39,3 @@
   background: #00d4ff;
   border: 2px solid #21262d;
 }
-.slider-thumb::-ms-thumb {
-  width: 13px;
-  height: 13px;
-  border-radius: 50%;
-  background: #00d4ff;
-  border: 2px solid #21262d;
-}