Unify process picker, improve notification CSS editor, remove notification led_count

- Extract shared process picker module (core/process-picker.js) used by
  both automation conditions and notification CSS app filter
- Remove led_count property from notification CSS source (backend + frontend)
- Replace comma-separated app filter with newline-separated textarea + browse
- Inline color cycle add button (+) into the color row
- Fix notification app color layout to horizontal rows

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

server/src/wled_controller/core/processing/notification_stream.py

@@ -70,8 +70,8 @@ class NotificationColorStripStream(ColorStripStream):
         self._app_colors = dict(getattr(source, "app_colors", {}))
         self._app_filter_mode = getattr(source, "app_filter_mode", "off")
         self._app_filter_list = list(getattr(source, "app_filter_list", []))
-        self._auto_size = not source.led_count
-        self._led_count = source.led_count if source.led_count and source.led_count > 0 else 1
+        self._auto_size = not getattr(source, "led_count", 0)
+        self._led_count = getattr(source, "led_count", 0) if getattr(source, "led_count", 0) > 0 else 1
         with self._colors_lock:
             self._colors: Optional[np.ndarray] = np.zeros((self._led_count, 3), dtype=np.uint8)