style(setup): address actionable golangci-lint findings

Fixes the lint hits that pointed at real bugs or dead code; leaves the
remaining style-only suggestions (rangeValCopy micro-copies, gocyclo
informational, intentional name choices like SetupStateMachine) alone.

- pkg/models/clockdisplay.go: restore <clockDisplay> XMLName tag on both
  ClockDisplay and ClockDisplayRequest. The earlier `xml:"-"` clashed
  with ClockDisplayUpdatedEvent.ClockDisplay's `xml:"clockDisplay"` tag
  (SA5008). Custom MarshalXML/UnmarshalXML still own the wire format.
- pkg/service/setup/setup.go: drop the now-unused checkRemoteServices
  helper (replaced by applyProbeToSummary) and rename the unused
  deviceIP parameter of populatePlannedNetworkConfig to _.
- pkg/service/setup/setup_session.go: collapse sendStep's (string, error)
  return to plain error — every caller already discarded the string.
- pkg/service/setup/init_plan.go: rename shadowed err variables to
  rwErr / genErr / invalidErr / nilErr / stepErr.
- cmd/soundtouch-cli/cmd_setup.go: drop redundant int(syscall.Stdin)
  conversion (already int) and rename a shadowed err to pairErr.

go build ./..., go vet ./..., and tests for the touched packages all
green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/soundtouch-cli/cmd_setup.go

@@ -626,7 +626,7 @@ func promptBasicAuth() (string, string, error) {
 
 	fmt.Fprint(os.Stderr, "Password: ")
 
-	pass, err := term.ReadPassword(int(syscall.Stdin))
+	pass, err := term.ReadPassword(syscall.Stdin)
 
 	fmt.Fprintln(os.Stderr)
 
@@ -1373,9 +1373,9 @@ func runPairBare(c *cli.Context, deviceIP, accountID string) error {
 
 	fmt.Printf("→ setMargeAccount accountID=%s (no SETUP bracket)\n", accountID)
 
-	if err := session.SetMargeAccount(ctx, accountID, ""); err != nil {
-		PrintError(fmt.Sprintf("setMargeAccount: %v", err))
-		return err
+	if pairErr := session.SetMargeAccount(ctx, accountID, ""); pairErr != nil {
+		PrintError(fmt.Sprintf("setMargeAccount: %v", pairErr))
+		return pairErr
 	}
 
 	time.Sleep(2 * time.Second)