Fix NaN timestamps with proper type checking

Previous || chain could pass through invalid values causing NaN.
Now explicitly checks each possible timestamp field with:
- null check (field != null)
- NaN check (!isNaN(parseFloat(field)))
- Takes first valid numeric value found

This ensures timestamps always have a valid number, defaulting
to 0 if no valid timestamp field is found.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

static/app.js

@@ -359,13 +359,23 @@
     segmentDiv.dataset.text = (segment.text || '').toLowerCase();
 
     // Handle multiple possible timestamp field names and formats
-    let startSeconds = segment.start_seconds
-                    || segment.start
-                    || segment.offset
-                    || segment.time
-                    || segment.timestamp
-                    || segment.startTime
-                    || 0;
+    // Try each field and ensure it's a valid number
+    let startSeconds = 0;
+    const possibleFields = [
+      segment.start_seconds,
+      segment.start,
+      segment.offset,
+      segment.time,
+      segment.timestamp,
+      segment.startTime
+    ];
+
+    for (const field of possibleFields) {
+      if (field != null && !isNaN(parseFloat(field))) {
+        startSeconds = parseFloat(field);
+        break;
+      }
+    }
 
     // If timestamp is in milliseconds (> 10000 for timestamps after ~2.7 hours), convert to seconds
     if (startSeconds > 10000) {