Fix Potential UI Deadlock

In `app.go`, the `applyAI` function locks a mutex and then spawns a goroutine that may try to lock the same mutex inside a `fyne.Do` block. If the UI thread is waiting on the mutex, this can cause a deadlock.

**Task:**

* [ ] Move the mutex lock inside the goroutine, so the UI thread never locks the mutex directly.

**Example Fix**

```go
u.setStatus("Contacting Gemini…")

go func() {
u.mu.Lock()
snapshot := append([]model.Packet(nil), u.packets...)
u.mu.Unlock()

filtered, err := u.chat.Reply(q, snapshot)
fyne.Do(func() {
// ... update UI ...
})
}()
```

**Why:**
This prevents the UI thread from being blocked and avoids deadlocks.