Center Window/Level preset pill text

The W/L preset pills (.preset-row label) had the same Gradio ml-2
span-margin offset as the body-region / voxel pills, and no rule was
zeroing it. Added .preset-row label > span { margin: 0 } — text is now
symmetric (13px/13px) in all four presets.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app.py

@@ -1284,6 +1284,8 @@ body [role="option"][aria-selected="true"] {
   transition: all 160ms ease;
 }
 .preset-row input[type="radio"] { display: none; }
+/* kill Gradio's ml-2 utility so W/L preset text is centered in its pill */
+.preset-row label > span { margin: 0 !important; }
 .preset-row label:has(input:checked) {
   background: color-mix(in srgb, var(--green) 14%, transparent) !important;
   border-color: var(--green) !important;