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/templates/modals/target-editor.html

@@ -45,6 +45,7 @@
                         <input type="range" id="target-editor-fps" min="1" max="90" value="30" oninput="document.getElementById('target-editor-fps-value').textContent = this.value">
                         <span class="slider-value">fps</span>
                     </div>
+                    <small id="target-editor-fps-rec" class="input-hint" style="display:none"></small>
                 </div>
 
                 <div class="form-group" id="target-editor-standby-group">