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/js/features/targets.js

@@ -12,7 +12,7 @@ import { API_BASE, getHeaders, fetchWithAuth, escapeHtml } from '../core/api.js'
 import { t } from '../core/i18n.js';
 import { showToast, showConfirm } from '../core/ui.js';
 import { Modal } from '../core/modal.js';
-import { createDeviceCard, attachDeviceListeners, fetchDeviceBrightness } from './devices.js';
+import { createDeviceCard, attachDeviceListeners, fetchDeviceBrightness, _computeMaxFps } from './devices.js';
 import { createKCTargetCard, connectKCWebSocket, disconnectKCWebSocket } from './kc-targets.js';
 import { createColorStripCard } from './color-strips.js';
 
@@ -93,6 +93,23 @@ function _autoGenerateTargetName() {
     document.getElementById('target-editor-name').value = `${deviceName} \u00b7 ${cssName}`;
 }
 
+function _updateFpsRecommendation() {
+    const el = document.getElementById('target-editor-fps-rec');
+    const deviceSelect = document.getElementById('target-editor-device');
+    const device = _targetEditorDevices.find(d => d.id === deviceSelect.value);
+    if (!device || !device.led_count) {
+        el.style.display = 'none';
+        return;
+    }
+    const fps = _computeMaxFps(device.baud_rate, device.led_count, device.device_type);
+    if (fps !== null) {
+        el.textContent = t('targets.fps.rec', { fps, leds: device.led_count });
+        el.style.display = '';
+    } else {
+        el.style.display = 'none';
+    }
+}
+
 function _updateStandbyVisibility() {
     const deviceSelect = document.getElementById('target-editor-device');
     const standbyGroup = document.getElementById('target-editor-standby-group');
@@ -167,12 +184,13 @@ export async function showTargetEditor(targetId = null) {
         // Auto-name generation
         _targetNameManuallyEdited = !!targetId;
         document.getElementById('target-editor-name').oninput = () => { _targetNameManuallyEdited = true; };
-        deviceSelect.onchange = () => { _updateStandbyVisibility(); _autoGenerateTargetName(); };
+        deviceSelect.onchange = () => { _updateStandbyVisibility(); _updateFpsRecommendation(); _autoGenerateTargetName(); };
         cssSelect.onchange = () => _autoGenerateTargetName();
         if (!targetId) _autoGenerateTargetName();
 
         // Show/hide standby interval based on selected device capabilities
         _updateStandbyVisibility();
+        _updateFpsRecommendation();
 
         targetEditorModal.snapshot();
         targetEditorModal.open();