DNS resolution chart is wrong

### Summary
DNS resolution times are occasionally displayed as negative values or null in the dashboard charts.

### Steps to Reproduce
1. Open the dashboard
2. Navigate to the DNS resolution chart
3. Observe the graph over time

### Expected Behavior
DNS resolution time should always be greater than or equal to 0 ms.

### Actual Behavior
- Some data points appear below 0 ms
- Graph shows null value

### Impact
- Misleading metrics
- Reduces confidence in measurement accuracy
- Makes it harder to interpret network performance

### Possible Causes (Speculation)
- Incorrect duration calculation
- Chart smoothing or interpolation artifact
- Null value not being properly handled

### Suggested Fix
- Ensure DNS resolution durations are always clamped to ≥ 0
- Review calculation logic for start/end timestamps
- Disable or adjust chart smoothing if applicable
- Show null value as 0 or as any other issue in network

### Environment
- OS: Windows 11 (amd64)
- Browser: Edge, Chrome
- Package: netmon windows amd64
- Release: v1.1.0