fix(tui): fill the frame background so the terminal backdrop can't bleed through

The main render path never applied the Screen backdrop, so any sub-view that left a line short or a row empty showed the transparent terminal background. Pad every frame to full width×height with the theme backdrop before display; content stays top-left anchored so caret coords are unaffected.

apps/tui-go/internal/app/view.go

@@ -41,14 +41,38 @@ const cursorMarker = ""
 // bubbletea v2 the View carries terminal state, so renderRaw() builds the string
 // and View() resolves the caret to a real blinking cursor + sets the window title.
 func (m Model) View() tea.View {
-	content, cur := splitCursor(m.renderRaw(), " ")
+	content, cur := splitCursor(m.fillFrame(m.renderRaw()), " ")
 	return tea.View{Content: content, AltScreen: true, Cursor: cur, WindowTitle: m.windowTitle()}
 }
 
+// fillFrame guarantees every cell of the width×height frame carries the theme backdrop, so a
+// sub-view that leaves a line short or a row empty shows the theme background rather than the
+// terminal-transparent backdrop bleeding through. Pads each line to full width and appends any
+// missing bottom rows; content stays anchored top-left so caret coordinates are unaffected.
+func (m Model) fillFrame(s string) string {
+	if m.width <= 0 || m.height <= 0 {
+		return s
+	}
+	pad := m.theme.Screen
+	lines := strings.Split(s, "\n")
+	out := make([]string, 0, m.height)
+	for i := 0; i < m.height; i++ {
+		ln := ""
+		if i < len(lines) {
+			ln = lines[i]
+		}
+		if gap := m.width - ansi.StringWidth(ln); gap > 0 {
+			ln += pad.Render(strings.Repeat(" ", gap))
+		}
+		out = append(out, ln)
+	}
+	return strings.Join(out, "\n")
+}
+
 // render returns the frame as a plain string with the caret drawn as the static
 // "▏" glyph — for the preview tool and golden tests, which have no live cursor.
 func (m Model) render() string {
-	content, _ := splitCursor(m.renderRaw(), "▏")
+	content, _ := splitCursor(m.fillFrame(m.renderRaw()), "▏")
 	return content
 }