Refactor watchdtui using the new bubbles Validate API

[GabrielNagy]

Describe the problem you'd like to have solved

The new bubbles release comes with a Validate API which we can use to validate our TUI inputs (directories and config file). Currently, validation is done via arbitrary functions that directly operate on the textinput.Model objects, e.g.

adsys/internal/watchdtui/watchdtui.go

Lines 397 to 435 in e298b39

	// updateConfigInputError updates the error state of the config input.
	func updateConfigInputError(input *textinput.Model) {
	value := input.Value()
	// If the config input is empty, clean up the error message
	if value == "" {
	input.Err = nil
	return
	}
	absPath, _ := filepath.Abs(value)
	stat, err := os.Stat(value)
	// If the config file does not exist, we're good
	if errors.Is(err, os.ErrNotExist) {
	input.Err = nil
	if !filepath.IsAbs(value) {
	input.Err = fmt.Errorf(i18n.G("%s will be the absolute path"), absPath)
	}
	return
	}
	// If we got another error, display it
	if err != nil {
	input.Err = err
	return
	}
	if stat.IsDir() {
	input.Err = fmt.Errorf(i18n.G("%s is a directory; will create %s.yaml inside"), absPath, watchdconfig.CmdName)
	return
	}
	if stat.Mode().IsRegular() {
	input.Err = fmt.Errorf(i18n.G("%s: file already exists and will be overwritten"), absPath)
	return
	}
	input.Err = nil
	}

Describe the ideal solution

Input validation is done via the newly added API.

[GabrielNagy]

I have a branch up for this refactor -- based on top of charmbracelet/bubbles#185

[jibel]

Fixed with 1f819dc