UI fixes: modal vertical scroll, hide overlay btn for non-picture CSS

- Modal content now constrained to viewport height with scrollable body,
  preventing dialogs from overflowing on small screens
- Overlay (👁️) button hidden for targets using static/gradient/color_cycle
  sources — calibration overlay only applies to picture-type sources

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/js/features/targets.js

@@ -514,7 +514,7 @@ export function createTargetCard(target, deviceMap, colorStripSourceMap) {
                 <button class="btn btn-icon btn-secondary" onclick="showTargetEditor('${target.id}')" title="${t('common.edit')}">
                     ✏️
                 </button>
-                ${state.overlay_active ? `
+                ${(!css || css.source_type === 'picture') ? (state.overlay_active ? `
                     <button class="btn btn-icon btn-warning" onclick="stopTargetOverlay('${target.id}')" title="${t('overlay.button.hide')}">
                         👁️
                     </button>
@@ -522,7 +522,7 @@ export function createTargetCard(target, deviceMap, colorStripSourceMap) {
                     <button class="btn btn-icon btn-secondary" onclick="startTargetOverlay('${target.id}')" title="${t('overlay.button.show')}">
                         👁️
                     </button>
-                `}
+                `) : ''}
             </div>
         </div>
     `;