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/switch.go

@@ -0,0 +1,95 @@
+package tui
+
+import (
+	tea "github.com/charmbracelet/bubbletea"
+
+	"submarine/internal/submodule"
+	"submarine/internal/tui/branches"
+)
+
+// branchesMsg is the branch list for the picker of path.
+type branchesMsg struct {
+	path     string
+	branches []submodule.Branch
+	err      error
+}
+
+// startSwitch opens the branch picker for the selection and reads its
+// branches.
+func (m Model) startSwitch() (tea.Model, tea.Cmd) {
+	s := m.tree.Selected()
+	sw, ok := m.provider.(submodule.Switcher)
+	var flash string
+	switch {
+	case s == nil:
+		return m, nil
+	case !ok:
+		flash = "this data source can't switch branches"
+	case m.restore != nil:
+		flash = "wait for the restore of " + m.restore.name + " to finish"
+	case !s.IsRoot() && s.Status != nil && s.Status.Presence != submodule.PresenceOK:
+		flash = s.Path + " is not initialized; restore it first"
+	}
+	if flash != "" {
+		cmd := m.footer.Flash(flash)
+		m.layout()
+		return m, cmd
+	}
+
+	m.picker, m.pickerPath = branches.New(m.theme, restoreName(s)), s.Path
+	m.picker.SetSpinner(m.spinner.View())
+	ctx, path := m.ctx, s.Path
+	load := func() tea.Msg {
+		bs, err := sw.Branches(ctx, path)
+		return branchesMsg{path, bs, err}
+	}
+	return m, tea.Batch(load, m.tick())
+}
+
+// branchesLoaded fills the picker, unless it was closed or is for another
+// repository meanwhile.
+func (m Model) branchesLoaded(msg branchesMsg) (tea.Model, tea.Cmd) {
+	if m.picker == nil || m.pickerPath != msg.path {
+		return m, nil
+	}
+	if msg.err != nil {
+		m.picker.SetErr(msg.err)
+	} else {
+		m.picker.SetBranches(msg.branches)
+	}
+	return m, nil
+}
+
+// pickBranch handles a key while the picker is open: arrows move, enter
+// switches, esc closes, everything else edits the search.
+func (m Model) pickBranch(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
+	switch msg.Type {
+	case tea.KeyCtrlC:
+		return m, tea.Quit
+	case tea.KeyEsc:
+		m.picker = nil
+	case tea.KeyUp:
+		m.picker.Up()
+	case tea.KeyDown:
+		m.picker.Down()
+	case tea.KeyPgUp:
+		m.picker.PageUp()
+	case tea.KeyPgDown:
+		m.picker.PageDown()
+	case tea.KeyEnter:
+		b := m.picker.Selected()
+		if b == nil {
+			return m, nil
+		}
+		s := m.byPath[m.pickerPath]
+		m.picker = nil
+		if s == nil || m.restore != nil {
+			return m, nil
+		}
+		m.restore = &restoreState{path: s.Path, name: restoreName(s), branch: b}
+		return m.checkRestore()
+	default:
+		return m, m.picker.Update(msg)
+	}
+	return m, nil
+}