Add switch branch (b): stage the tip and restore to it

b opens a centered popup with a search line and the local and remote
branches of the selection. Enter stages the branch tip as the pin of a
submodule, restores it (and everything below) to it, and puts HEAD on the
branch; a remote branch gets a local tracking branch unless one exists
elsewhere. Works on the superproject too. Losses are asked like restore.
The backdrop behind the popup is dimmed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/tui/overlay.go

@@ -0,0 +1,34 @@
+package tui
+
+import (
+	"strings"
+
+	"github.com/charmbracelet/lipgloss"
+	"github.com/charmbracelet/x/ansi"
+)
+
+// overlay draws fg centered over bg, which is w x h cells. bg must be the
+// final string (after zone.Scan): its lines are cut at the popup's edges.
+// The backdrop loses its colors and is drawn in dim, so the popup stands out.
+func overlay(bg, fg string, w, h int, dim lipgloss.Style) string {
+	lines := strings.Split(bg, "\n")
+	for i, l := range lines {
+		lines[i] = dim.Render(ansi.Strip(l))
+	}
+	pop := strings.Split(fg, "\n")
+	pw := lipgloss.Width(fg)
+	top := max((h-len(pop))/2, 0)
+	left := max((w-pw)/2, 0)
+	for i, p := range pop {
+		y := top + i
+		if y >= len(lines) {
+			break
+		}
+		line := lines[y]
+		if lw := ansi.StringWidth(line); lw < left {
+			line += strings.Repeat(" ", left-lw)
+		}
+		lines[y] = ansi.Truncate(line, left, "") + "\x1b[0m" + p + "\x1b[0m" + ansi.TruncateLeft(line, left+pw, "")
+	}
+	return strings.Join(lines, "\n")
+}