fix: render full placeholder text in form inputs

bubbles/v2 textinput truncates the placeholder to its first rune when the
input width is 0 (the default): it sizes the placeholder buffer to
Width()+1 and early-returns after the first character. Set a non-zero
width on every text input (defaultInputWidth) and resize inputs
responsively to the terminal on WindowSizeMsg via FormStep.SetWidth, so
placeholders render in full and inputs fill the available width.

Adds a regression test asserting the placeholder renders past its first
rune.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/tui/form.go

@@ -17,6 +17,11 @@ const (
 	FieldTypeNumber
 )
 
+// defaultInputWidth is the visible width of text inputs before the first window-size
+// message arrives. It must be > 0 (and ideally >= the longest placeholder) so the
+// placeholder renders in full; it is overridden responsively via FormStep.SetWidth.
+const defaultInputWidth = 50
+
 type FormField struct {
 	Id          string
 	Label       string
@@ -51,6 +56,9 @@ func NewFormStep(title string, fields []FormField) FormStep {
 
 		ti := textinput.New()
 		ti.Placeholder = f.Fields[i].Placeholder
+		// A non-zero width is required for the placeholder to render in full: with the
+		// default width of 0 the textinput truncates the placeholder to its first rune.
+		ti.SetWidth(defaultInputWidth)
 		if f.Fields[i].Default != "" {
 			ti.SetValue(f.Fields[i].Default)
 		}
@@ -129,6 +137,17 @@ func (f *FormStep) Values() map[string]string {
 	return out
 }
 
+// SetWidth resizes every text input to w (selects are unaffected). Called on window
+// resize so inputs fill the available terminal width and placeholders render in full.
+func (f *FormStep) SetWidth(w int) {
+	for i := range f.Fields {
+		if f.Fields[i].Type == FieldTypeSelect {
+			continue
+		}
+		f.Fields[i].input.SetWidth(w)
+	}
+}
+
 // View
 func (f FormStep) View() string {
 	pad := strings.Repeat(" ", padding)