Support the mouse in the branch popup

A click on a branch switches to it, the wheel moves the selection, a click
outside closes the popup. The popup is hit-tested by its own position, as it
is drawn after zone.Scan.

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

docs/architecture.md

@@ -28,7 +28,7 @@ internal/tui/                 Bubble Tea application
   app.go                      root model: layout, message routing, loading
   keys.go                     key bindings (also feed the help view)
   restore.go, switch.go       restore / switch flows; a switch reuses restoreState
-  overlay.go                  draws the picker centered over the finished view
+  overlay.go                  draws the picker over the finished view at the position the picker reports
   msgs.go                     tree/status/fetch/change messages
   refresh.go                  reloader: one load per key, coalesced reruns
   theme/                      colors, glyphs, issue words, change letters, level styles