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/keys.go

@@ -7,7 +7,8 @@ type keyMap struct {
 	CollapseAll, ExpandAll        key.Binding
 	Home, End, PageUp, PageDown   key.Binding
 	Fetch, Reload, Filter, Issues key.Binding
-	Copy, Restore, Help, Quit     key.Binding
+	Copy, Restore, Switch         key.Binding
+	Help, Quit                    key.Binding
 }
 
 func defaultKeys() keyMap {
@@ -28,6 +29,7 @@ func defaultKeys() keyMap {
 		Filter:      key.NewBinding(key.WithKeys("/"), key.WithHelp("/", "filter")),
 		Issues:      key.NewBinding(key.WithKeys("!"), key.WithHelp("!", "problems only")),
 		Restore:     key.NewBinding(key.WithKeys("x"), key.WithHelp("x", "restore")),
+		Switch:      key.NewBinding(key.WithKeys("b"), key.WithHelp("b", "switch branch")),
 		Copy:        key.NewBinding(key.WithKeys("c"), key.WithHelp("c", "copy path")),
 		Help:        key.NewBinding(key.WithKeys("?"), key.WithHelp("?", "help")),
 		Quit:        key.NewBinding(key.WithKeys("q", "ctrl+c"), key.WithHelp("q", "quit")),
@@ -44,7 +46,7 @@ func (k keyMap) Navigation() []key.Binding {
 
 // Submodule returns the bindings that act on the selected submodule. They
 // are shown in the detail pane, or in the footer if the pane can't show them.
-func (k keyMap) Submodule() []key.Binding { return []key.Binding{k.Copy, k.Restore} }
+func (k keyMap) Submodule() []key.Binding { return []key.Binding{k.Copy, k.Switch, k.Restore} }
 
 // Global returns the bindings that don't depend on the selection.
 func (k keyMap) Global() []key.Binding { return []key.Binding{k.Help, k.Quit} }
@@ -58,7 +60,7 @@ func (k keyMap) FullHelp() [][]key.Binding {
 		{k.Up, k.Down, k.Left, k.Right, k.Toggle},
 		{k.CollapseAll, k.ExpandAll},
 		{k.Home, k.End, k.PageUp, k.PageDown},
-		{k.Copy, k.Restore},
+		{k.Copy, k.Switch, k.Restore},
 		{k.Fetch, k.Reload, k.Filter, k.Issues},
 		{k.Help, k.Quit},
 	}