feat: enhance waveform viewer with record type handling and improved empty state messaging

sfm/waveform_viewer.html

@@ -374,13 +374,7 @@
     const decoded   = data.samples_decoded || 0;
     const total     = data.total_samples   || decoded;
     const channels  = data.channels || {};
-
-    // Build time axis (ms)
-    const times = Array.from({ length: decoded }, (_, i) =>
-      ((i - pretrig) / sr * 1000).toFixed(2)
-    );
-
-    const triggerMs = 0;  // t=0 is trigger by construction
+    const recType   = data.record_type || 'Unknown';
 
     // Status bar
     const bar = document.getElementById('status-bar');
@@ -392,11 +386,30 @@
     } else {
       bar.textContent = `Event #${data.index}    `;
     }
+    appendMeta('type', recType);
     appendMeta('sr', `${sr} sps`);
     appendMeta('samples', `${decoded.toLocaleString()} / ${total.toLocaleString()}`);
     appendMeta('pretrig', pretrig);
     appendMeta('rectime', `${data.rectime_seconds ?? '?'}s`);
 
+    // No waveform data — show a clear reason instead of empty charts
+    if (decoded === 0) {
+      document.getElementById('empty-state').style.display = 'flex';
+      document.getElementById('empty-state').querySelector('p').textContent =
+        recType === 'Waveform'
+          ? 'Waveform decode returned no samples — check server logs'
+          : `Record type "${recType}" — waveform decode not yet supported for this mode`;
+      document.getElementById('charts').style.display = 'none';
+      Object.values(charts).forEach(c => c.destroy());
+      charts = {};
+      return;
+    }
+
+    // Build time axis (ms)
+    const times = Array.from({ length: decoded }, (_, i) =>
+      ((i - pretrig) / sr * 1000).toFixed(2)
+    );
+
     // Show charts area
     document.getElementById('empty-state').style.display = 'none';
     const chartsDiv = document.getElementById('charts');