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.tsx

@@ -235,9 +235,8 @@ export function GroupRectModal({ open, onClose, onSubmit, onDelete, initial, tit
                       max={100}
                       value={alpha}
                       onChange={(e) => set(key, rgbaToHex8(hex6, Number(e.target.value)))}
-                      className={`w-full h-1 cursor-pointer mt-2 ${styles['slider-thumb']} ${styles['slider-accent']}`}
+                      className={`w-full cursor-pointer mt-2 ${styles['slider-thumb']} ${styles['slider-accent']}`}
                       title={`Opacity: ${alpha}%`}
-                      style={{ accentColor: '#00d4ff' }}
                     />
                     <span className="text-[9px] text-muted-foreground/60">{label} {alpha}%</span>
                   </div>