Animation None option, FPS min 1, serial COM lifecycle fixes

- Replace animation Enable checkbox with None option in effect selector;
  show effect description tooltip; disable speed slider when None selected
- Allow target FPS range 1-90 (was 10-90) across UI and backend validation
- Scope serial COM connections to target lifetime (no idle caching);
  use temporary connections for power-off/test mode
- Fix serial black frame on stop: flush after write, delay after task
  cancel to prevent race with in-flight thread pool write

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

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

@@ -203,6 +203,14 @@
     font-size: 0.85rem;
 }
 
+.field-desc {
+    display: block;
+    margin: 4px 0 0 0;
+    color: #888;
+    font-size: 0.82rem;
+    font-style: italic;
+}
+
 .fps-hint {
     display: block;
     margin-top: 4px;