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/storage/wled_picture_target.py

@@ -19,7 +19,7 @@ class WledPictureTarget(PictureTarget):
 
     device_id: str = ""
     color_strip_source_id: str = ""
-    fps: int = 30                   # target send FPS (10-90)
+    fps: int = 30                   # target send FPS (1-90)
     standby_interval: float = 1.0   # seconds between keepalive sends when screen is static
     state_check_interval: int = DEFAULT_STATE_CHECK_INTERVAL