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_test.go

@@ -1,6 +1,23 @@
 package tui
 
-import "testing"
+import (
+	"strings"
+	"testing"
+)
+
+// Guards against the bubbles bug where a text input with width 0 renders only the
+// first rune of its placeholder. NewFormStep must set a non-zero width.
+func TestPlaceholderRendersBeyondFirstRune(t *testing.T) {
+	const placeholder = "postgres://user@host/db"
+	f := NewFormStep("t", []FormField{
+		{Id: "url", Label: "URL", Placeholder: placeholder, Type: FieldTypeText},
+	})
+
+	view := f.Fields[0].input.View()
+	if !strings.Contains(view, "ostgres://user@host/db") {
+		t.Errorf("placeholder appears truncated; rendered view = %q", view)
+	}
+}
 
 func TestFormStepValues(t *testing.T) {
 	f := NewFormStep("Test", []FormField{