fix: default to flatline when no live waveform data is received (enable with demo=true)

fiveparaminte-main/static/dashboard.html

@@ -255,6 +255,7 @@ const params = new URLSearchParams(location.search);
 const MRN      = params.get('mrn') || '';
 const isMinimal = params.get('minimal') === 'true' || params.get('view') === 'minimal';
 const DEVICE_PARAM = params.get('device') || '';
+const isDemo = params.get('demo') === 'true' || params.get('synth') === 'true';
 
 if (isMinimal) document.body.classList.add('minimal-mode');
 
@@ -346,7 +347,19 @@ class WaveformTrack {
       this.drawBuf[(this.sweepPos + e) % w] = null;
     }
 
-    const yNorm = this.queue.length > 0 ? this.queue.shift() : this._synth();
+    let yNorm;
+    if (this.queue.length > 0) {
+      yNorm = this.queue.shift();
+    } else if (isDemo) {
+      yNorm = this._synth();
+    } else {
+      // Flatline baseline based on mode
+      if (this.mode === 'pleth') {
+        yNorm = 0.85; // flatline at bottom for pleth
+      } else {
+        yNorm = 0.5;  // center flatline for ecg/resp
+      }
+    }
     this.drawBuf[this.sweepPos] = yNorm;
     this.sweepPos = (this.sweepPos + 1) % w;
   }