CSS: add led_count field; calibration dialog improvements; color corrections collapsible section

- Add explicit led_count to PictureColorStripSource (0 = auto from calibration)
- Stream pads with black or truncates to match led_count exactly
- Calibration dialog: show led_count input above visual editor in CSS mode
- Calibration dialog: pre-populate led_count with effective count (cal sum) when stored value is 0
- Calibration dialog: sync preview label live as led_count input changes
- CSS editor: group brightness/saturation/gamma into collapsible "Color Corrections" section

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

server/src/wled_controller/static/css/modal.css

@@ -221,6 +221,47 @@
     color: var(--text-primary);
 }
 
+.form-collapse {
+    margin-bottom: 12px;
+    border-top: 1px solid var(--border-color);
+    padding-top: 8px;
+}
+
+.form-collapse > summary {
+    cursor: pointer;
+    list-style: none;
+    display: flex;
+    align-items: center;
+    gap: 6px;
+    font-size: 0.9rem;
+    font-weight: 500;
+    color: var(--text-secondary, #888);
+    padding: 4px 0;
+    user-select: none;
+}
+
+.form-collapse > summary::-webkit-details-marker { display: none; }
+
+.form-collapse > summary::before {
+    content: '▶';
+    font-size: 0.6rem;
+    opacity: 0.6;
+    transition: transform 0.15s;
+    flex-shrink: 0;
+}
+
+.form-collapse[open] > summary::before {
+    transform: rotate(90deg);
+}
+
+.form-collapse > summary:hover {
+    color: var(--text-color);
+}
+
+.form-collapse-body {
+    padding-top: 8px;
+}
+
 .error-message {
     background: rgba(244, 67, 54, 0.1);
     border: 1px solid var(--danger-color);