Fix PlayNotificationBeep to use GET instead of POST

- Fixed HTTP method mismatch: /playNotification endpoint expects GET, not POST
- Updated PlayNotificationBeep() to use existing c.get() method with StationResponse model
- Resolves HTTP 400 errors when using 'soundtouch-cli sp beep' command
- Verified working with SoundTouch 20 hardware
- Added comprehensive troubleshooting documentation
- Updated feature history with bug fix details

Fixes: go run ./cmd/soundtouch-cli --host <device> sp beep
Previously failed with: 'API request failed with status 400'
Now works correctly alongside: curl http://<device>:8090/playNotification

pkg/client/client.go

@@ -1677,7 +1677,8 @@ func (c *Client) PlayCustom(playInfo *models.PlayInfo) error {
 
 // PlayNotificationBeep plays a notification beep on the device
 func (c *Client) PlayNotificationBeep() error {
-	return c.post("/playNotification", nil)
+	var status models.StationResponse
+	return c.get("/playNotification", &status)
 }
 
 // postPlayInfo sends a PlayInfo request to the /speaker endpoint