Show max FPS hint in target editor, fix gradient sharing for multi-target

- Add dynamic "Hardware max ≈ N fps" recommendation below FPS slider,
  computed from LED count (WLED: protocol timing) or baud rate (serial).
  Reuses shared _computeMaxFps from devices.js with named constants.
- Fix gradient looking different across targets sharing the same stream:
  configure() now uses max LED count across all consumers; _fit_to_device
  uses np.interp linear interpolation instead of truncate/tile.

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

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

@@ -362,6 +362,7 @@
   "targets.source.none": "-- No source assigned --",
   "targets.fps": "Target FPS:",
   "targets.fps.hint": "Target frames per second for capture and LED updates (1-90)",
+  "targets.fps.rec": "Hardware max ≈ {fps} fps ({leds} LEDs)",
   "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:",