Add a "Clear Filters" Button

Currently, there is no way to reset all filter input fields in the UI at once.

**Task:**

* [ ] Add a "Clear" button to the filter section of the UI (in `app.go`).
* [ ] When clicked, the button should reset all filter fields and re-apply filters to show all packets.

**Example:**

```go
clear := widget.NewButton("Clear", func() {
u.filterProtocol.SetText("")
u.filterSrcIP.SetText("")
u.filterDstIP.SetText("")
u.filterSrcPort.SetText("")
u.filterDstPort.SetText("")
u.filterFreeText.SetText("")
u.applyFilters()
})
```

**Why:**
This will improve usability for users who want to quickly reset their search.