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/locales/en.json

@@ -1,6 +1,7 @@
 {
   "app.title": "LED Grab",
   "app.version": "Version:",
+  "app.api_docs": "API Documentation",
   "theme.toggle": "Toggle theme",
   "locale.change": "Change language",
   "auth.login": "Login",
@@ -370,6 +371,8 @@
   "kc.error.no_pattern": "Please select a pattern template",
   "kc.error.required": "Please fill in all required fields",
   "kc.colors.none": "No colors extracted yet",
+  "kc.test": "Test",
+  "kc.test.error": "Test failed",
   "targets.section.pattern_templates": "📄 Pattern Templates",
   "pattern.add": "📄 Add Pattern Template",
   "pattern.edit": "📄 Edit Pattern Template",