fix(spe): drop the link on power-off so status shows off and ON works

Switching an SPE amp off left the already-open connection reading as connected
(the amp stops answering once off, but the stale open handle kept the status at
"connected"), so the panel never showed OFF and PowerOn — which only wakes a
DISCONNECTED amp, and must NOT pulse RTS/DTR on a running one — did nothing
until the operator restarted OpsLog. PowerOff now drops the connection and marks
the amp offline, exactly as a fresh start would; the poll's plain reopen never
wakes a switched-off amp, so it stays off until the ON button's RTS/DTR edge.

internal/spe/spe.go

@@ -228,6 +228,18 @@ func (c *Client) PowerOn() error {
 func (c *Client) PowerOff() error {
 	err := c.sendCmd(cmdOff)
 	applog.Printf("spe: power OFF — SWITCH OFF key sent (err=%v)", err)
+	// Drop the link and mark the amp offline at once, exactly as a fresh start
+	// would see it. The amp stops answering its UART once off, but the connection
+	// that was already open BEFORE the off keystroke kept reading as "connected"
+	// (stale/queued frames), so the UI never showed OFF and PowerOn — which only
+	// wakes a DISCONNECTED amp — did nothing until the operator restarted OpsLog.
+	// The poll loop reopens the port on its next tick; a plain reopen never wakes a
+	// switched-off amp (only PowerOn's RTS/DTR edge does), so it stays offline.
+	time.Sleep(100 * time.Millisecond) // let the 6-byte OFF command flush before we close
+	c.mu.Lock()
+	c.dropLocked()
+	c.mu.Unlock()
+	c.setErr(fmt.Errorf("switched off"))
 	return err
 }