fix(ui): edit-mode wizard allows keeping the same local port

Previously, editing a forward and keeping its local port unchanged
failed the wizard's port-availability check: the in-config scan
found the forward's own entry and reported '✗ Port N already
assigned to <self.ID>'. Users had to pick a different port,
edit, then change back.

checkPortCmd now accepts an excludeID. The wizard passes
wizard.originalID when isEditing so the forward being edited is
ignored during the in-config conflict scan. The OS-level port
check is unchanged (still catches actual port collisions).

New regression test: TestCheckPortCmd_ExcludeID_AllowsKeepingOwnPort.

internal/ui/wizard_handlers.go

@@ -656,7 +656,11 @@ func (m model) handleAddWizardEnter() (tea.Model, tea.Cmd) {
 			wizard.localPort = port
 			wizard.loading = true
 			wizard.error = nil
-			return m, checkPortCmd(port, m.ui.configPath)
+			excludeID := ""
+			if wizard.isEditing {
+				excludeID = wizard.originalID
+			}
+			return m, checkPortCmd(port, m.ui.configPath, excludeID)
 		}
 
 	case StepConfirmation: