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/css/modal.css

@@ -24,6 +24,9 @@
     border-radius: 12px;
     max-width: 500px;
     width: 90%;
+    max-height: calc(100vh - 40px);
+    display: flex;
+    flex-direction: column;
     box-shadow: 0 8px 32px rgba(0, 0, 0, 0.5);
     animation: slideUp 0.3s ease-out;
 }
@@ -113,6 +116,10 @@
 
 .modal-body {
     padding: 24px;
+    overflow-y: auto;
+    scrollbar-gutter: stable;
+    flex: 1 1 auto;
+    min-height: 0;
 }
 
 .modal-description {