Add KC target test button, API docs header link, and UI polish

- Add POST /api/v1/picture-targets/{target_id}/test endpoint for single-frame
  color extraction preview on Key Colors targets
- Add test button on KC target cards that opens lightbox with spinner,
  displays captured frame with rectangle overlays and color swatches
- Add API docs link in WebUI header
- Swap confirm dialog button colors (No=red, Yes=neutral)
- Remove type badges from WLED and KC target cards

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

server/src/wled_controller/static/style.css

@@ -91,6 +91,21 @@ h2 {
     gap: 15px;
 }
 
+.header-link {
+    color: var(--text-secondary);
+    text-decoration: none;
+    font-size: 0.85rem;
+    font-weight: 500;
+    padding: 4px 8px;
+    border-radius: 4px;
+    transition: color 0.2s, background 0.2s;
+}
+
+.header-link:hover {
+    color: var(--text-color);
+    background: var(--bg-secondary);
+}
+
 #server-version {
     font-size: 0.75rem;
     font-weight: 400;