Update a pin that is behind with u; switch fast-forwards

`u` moves the pin of the selection to the tip of its ref when the pin is
only behind it: a switch to that branch with exactly the tip the row
shows, nothing fetched; if the ref moved meanwhile, nothing happens. On
the superproject it fast-forwards the branch to its upstream and restores
the submodules (git pull --ff-only from the last fetch).

Switch now fast-forwards a local branch that is only behind the chosen
remote branch, unless another worktree has it checked out, and keeps a
checked-out branch that tracks it under another name. A branch with
commits of its own is still never moved.

Stage pin is dropped: switching to the branch already stages it. The
detail pane's palette wraps instead of cutting off keys. Add D29 and F59.

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

internal/tui/app.go

@@ -451,6 +451,8 @@ func (m Model) handleKey(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 		return m.startRestore()
 	case key.Matches(msg, k.Switch):
 		return m.startSwitch()
+	case key.Matches(msg, k.Update):
+		return m.startPinUpdate()
 	case key.Matches(msg, k.Issues):
 		m.tree.SetProblemsOnly(!m.tree.ProblemsOnly())
 		m.layout()