feat: make global default timeout 0 (infinite) instead of required 30s

DefaultTimeoutSeconds now means "no timeout" when 0/empty, and that is
the new default, rather than an invalid config forcing a positive
value. runner.RunJob avoids context.WithTimeout with a zero duration
(which would expire immediately) and instead runs on a plain
cancelable context when no timeout is configured. Per-job
TimeoutSeconds inherit semantics are unchanged.

src/ui/settings_view.go

@@ -86,6 +86,7 @@ func settingsView(w fyne.Window, svc *app.Service) fyne.CanvasObject {
 	overlapPolicySelect.SetSelected(string(store.Config.OverlapPolicy))
 	overlapPolicySelect.OnChanged = func(string) { updateSaveState() }
 	defaultTimeout := widget.NewEntry()
+	defaultTimeout.SetPlaceHolder("0 = no timeout")
 	defaultTimeout.SetText(strconv.Itoa(store.Config.DefaultTimeoutSeconds))
 	defaultTimeout.OnChanged = func(string) { updateSaveState() }
 	jobsDir := widget.NewEntry()
@@ -132,8 +133,8 @@ func settingsView(w fyne.Window, svc *app.Service) fyne.CanvasObject {
 			return
 		}
 		timeout, err := strconv.Atoi(strings.TrimSpace(defaultTimeout.Text))
-		if err != nil || timeout <= 0 {
-			settingsStatus.SetText("Default timeout must be a positive number")
+		if err != nil || timeout < 0 {
+			settingsStatus.SetText("Default timeout must not be negative (0 = no timeout)")
 			return
 		}
 		// Build the new config from the form and hand it to the Service, which