docs: add refinement span hint in UI

web/index.html

@@ -252,6 +252,7 @@
                 <label class="field"><span>Min Span (Hz)</span><input id="refineMinSpan" type="number" step="500" min="0" /></label>
                 <label class="field"><span>Max Span (Hz)</span><input id="refineMaxSpan" type="number" step="500" min="0" /></label>
               </div>
+              <div class="panel-sub">Auto span uses modulation hints when candidate BW is missing. Min/Max clamp the final window.</div>
             </div>
           </section>