Upgrade MyGo to v0.5.5

MyGo v0.5.5 returns ui.Element as a checked value handle instead of a
pointer. Migrated with `mygo migrate-ui`, then fixed what the codemod
leaves to review:

- Element nil checks become .Valid(); absent elements are ui.Element{}.
- Input handlers and paint callbacks run outside the build pass, where
  elements expire and Context methods are inert: read IsDisabled into a
  local during the build, keep a ui.Handle for Focus, use c.Services()
  for clipboard access, create ui.Local state during the build.
- Add core.AutoFocus: v0.5.5 records an overlay's opener after its
  content builds, so same-frame autofocus lost focus restore on close.
  It focuses one frame later; cascader rows defer their focus likewise.
- core.OverlayLayer identifies build passes by the root element, not
  c.Now(), which repeats across passes and under the Tester clock.
- Context menus survive the opening press counted as PressedOutside;
  split button tooltip no longer reorders its open menu; disabled
  menubar rows pass presses to the panel; Form defers blur-started
  async checks so the growing message no longer moves Submit away
  from the release; Combobox drops its disabled copy now that MyGo
  blocks typing under a disabled wrapper (canary flipped to a guard).
- The Tester clock is virtual: tests use Advance/Now instead of
  time.Sleep/time.Now and read element state inside the view. The web
  mirror advances the clock to wall time before each frame and input.
- AGENTS.md and docs describe the value Element API and the new pin.

Snapshot goldens are darwin-only and were not re-verified here.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

messaging/status_setter.go

@@ -99,7 +99,7 @@ func StatusSetter(c *ui.Context, st *CustomStatus, opts StatusSetterOptions) cor
 			for i, o := range statusClear {
 				options[i] = input.SelectOption[int]{Value: i, Label: core.Msg(c, o.key)}
 			}
-			ui.Box(c).Key(live).Children(func() {
+			ui.Box(c.Key(live)).Children(func() {
 				if input.Select(c, &s.clear, options, input.SelectOptions{Label: core.Msg(c, "status_setter.clearAfter"), Placeholder: core.MessagingDate(c, st.Until.In(loc), "status_setter.until"), Disabled: !live}).Changed() {
 					st.Until = statusUntil(c.Now().In(loc), statusClear[s.clear].after)
 				}