feat(tui-go): transparent modal backdrops

Modals now composite over a dimmed copy of the screen behind them instead of an opaque
scrim, so the transcript stays faintly visible around a modal (opencode-style). center()
stashes the frame's base (View sets m.lastBase), strips+dims it to a faint scrim, and
splices the modal box over it (ANSI-aware via ansi.Truncate/TruncateLeft so the side
margins keep the dimmed content). An idempotence guard passes an already-full-screen
modal through unchanged, so the existing double-center builders don't get re-dimmed.
Geometry tests assert the composite still fills the screen exactly.

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

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

@@ -9,13 +9,74 @@ import (
 	"github.com/charmbracelet/x/ansi"
 )
 
-// center composites a modal over a scrim-filled screen. Immediate-mode: the modal
-// is recomputed from state every frame, so there is no separate show/restore path
-// to desync (the "diff won't come back" bug class can't occur here).
+// center draws a modal centered over a *dimmed* copy of the screen behind it, so the
+// transcript stays faintly visible around the modal (a transparent backdrop) rather
+// than being hidden by an opaque scrim. Immediate-mode: recomputed from state every
+// frame, so there is no show/restore path to desync.
+//
+// Idempotence guard: some modal builders call center() internally and then get centered
+// again by renderOverlay. The inner call already produced a full-screen composite, so a
+// second call (input already width×height) returns it unchanged instead of dimming the
+// modal itself. Falls back to an opaque scrim when no base was stashed (defensive).
 func (m Model) center(modal string) string {
-	return lipgloss.Place(m.width, m.height, lipgloss.Center, lipgloss.Center, modal,
-		lipgloss.WithWhitespaceBackground(m.theme.P.BgDeep),
-		lipgloss.WithWhitespaceForeground(m.theme.P.BgDeep))
+	if m.lastBase == "" {
+		return lipgloss.Place(m.width, m.height, lipgloss.Center, lipgloss.Center, modal,
+			lipgloss.WithWhitespaceBackground(m.theme.P.BgDeep),
+			lipgloss.WithWhitespaceForeground(m.theme.P.BgDeep))
+	}
+	if lipgloss.Height(modal) >= m.height && lipgloss.Width(modal) >= m.width {
+		return modal // already a full-screen composite; don't re-dim it
+	}
+	return m.compositeOverlay(m.lastBase, modal)
+}
+
+// compositeOverlay strips + dims base into a faint scrim, then splices the modal box
+// over it centered. ANSI-aware so the side margins keep the (dimmed) content behind.
+// base and the result are both m.width × m.height.
+func (m Model) compositeOverlay(base, modal string) string {
+	w, h := m.width, m.height
+	dim := lipgloss.NewStyle().Foreground(m.theme.P.Faint).Background(m.theme.P.BgDeep)
+	baseLines := strings.Split(base, "\n")
+	scrim := make([]string, h)
+	for i := 0; i < h; i++ {
+		raw := ""
+		if i < len(baseLines) {
+			raw = ansi.Strip(baseLines[i])
+		}
+		scrim[i] = dim.Render(padRightRaw(raw, w))
+	}
+	modalLines := strings.Split(modal, "\n")
+	mw := lipgloss.Width(modal)
+	top := (h - len(modalLines)) / 2
+	left := (w - mw) / 2
+	if top < 0 {
+		top = 0
+	}
+	if left < 0 {
+		left = 0
+	}
+	for r, ml := range modalLines {
+		row := top + r
+		if row < 0 || row >= h {
+			continue
+		}
+		leftPart := ansi.Truncate(scrim[row], left, "")
+		rightPart := ansi.TruncateLeft(scrim[row], left+mw, "")
+		scrim[row] = leftPart + ml + rightPart
+	}
+	return strings.Join(scrim, "\n")
+}
+
+// padRightRaw pads an unstyled string with spaces to exactly w columns (or truncates).
+func padRightRaw(s string, w int) string {
+	switch sw := ansi.StringWidth(s); {
+	case sw > w:
+		return ansi.Truncate(s, w, "")
+	case sw < w:
+		return s + strings.Repeat(" ", w-sw)
+	default:
+		return s
+	}
 }
 
 func (m Model) modalWidth() int {