feat(tui-go): idea board viewer

OverlayIdeas (opened with I, global/cross-session) lists the idea board in the
artifact-viewer shell: ↑↓ to move, x/d to remove (sends DiscardIdea, optimistic
drop), esc to close. Pull-based via ListIdeas -> idea.list.

apps/tui-go/internal/app/overlays.go

@@ -59,6 +59,8 @@ func (m Model) renderOverlay(base string) string {
 		return m.center(m.configModal())
 	case OverlayStats:
 		return m.center(m.statsModal())
+	case OverlayIdeas:
+		return m.center(m.ideasModal())
 	}
 	return base
 }