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/update.go

@@ -33,6 +33,19 @@ func (m Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 	case tea.WindowSizeMsg:
 		m.width = msg.Width
 		m.height = msg.Height
+
+		// Fit inputs to the terminal: subtract the left padding, the "> " prompt and
+		// the cursor cell, with a sensible floor for very narrow terminals.
+		inputWidth := msg.Width - (padding * 2) - 4
+		if inputWidth < 20 {
+			inputWidth = 20
+		}
+		for _, f := range []*FormStep{
+			&m.loginForm, &m.wireguardForm, &m.appForm,
+			&m.serverForm, &m.dbForm, &m.certForm,
+		} {
+			f.SetWidth(inputWidth)
+		}
 	case spinner.TickMsg:
 		var cmd tea.Cmd
 		m.spinner, cmd = m.spinner.Update(msg)