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>

chat/conversation_export.go

@@ -146,7 +146,7 @@ type ConversationExportOptions struct {
 
 // ConversationExportView is a built ConversationExport and the export asked for.
 type ConversationExportView struct {
-	Element *ui.Element
+	Element ui.Element
 	// Exported reports Content is ready for the caller to write.
 	Exported bool
 	Content  string
@@ -172,13 +172,13 @@ func ConversationExport(c *ui.Context, s *ConversationExportState, messages []Se
 	v := ConversationExportView{Element: col, Format: s.Format}
 	content := conversationExportContent(s, messages, opts)
 	col.Children(func() {
-		input.FormField(c, core.Msg(c, "conversation_export.format"), input.FormFieldOptions{}, func() *ui.Element {
+		input.FormField(c, core.Msg(c, "conversation_export.format"), input.FormFieldOptions{}, func() ui.Element {
 			return input.RadioGroup(c, &s.Format, []input.RadioItem[ExportFormat]{
 				{Value: ExportMarkdown, Label: "Markdown"}, {Value: ExportJSON, Label: "JSON"},
 				{Value: ExportText, Label: core.Msg(c, "conversation_export.text")},
 			}, input.RadioGroupOptions{Horizontal: true, Label: core.Msg(c, "conversation_export.format"), Disabled: opts.Disabled}).Element
 		})
-		input.FormField(c, core.Msg(c, "conversation_export.range"), input.FormFieldOptions{}, func() *ui.Element {
+		input.FormField(c, core.Msg(c, "conversation_export.range"), input.FormFieldOptions{}, func() ui.Element {
 			return ui.Row(c).Gap(d.Unit() * 3).AlignItems(ui.Center).Children(func() {
 				input.RadioGroup(c, &s.Range, []input.RadioItem[ExportRange]{
 					{Value: ExportAll, Label: fmt.Sprintf(core.Msg(c, "conversation_export.all"), len(messages))},