Add Target FPS slider to WLED target editor dialog

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

server/src/wled_controller/static/locales/en.json

@@ -318,6 +318,8 @@
   "targets.source": "Source:",
   "targets.source.hint": "Which picture source to capture and process",
   "targets.source.none": "-- No source assigned --",
+  "targets.fps": "Target FPS:",
+  "targets.fps.hint": "Target frames per second for capture and LED updates (10-90)",
   "targets.border_width": "Border Width (px):",
   "targets.border_width.hint": "How many pixels from the screen edge to sample for LED colors (1-100)",
   "targets.interpolation": "Interpolation Mode:",