motion: toasts animate out; the welcome tour slides in the direction of travel (#1645)

[ZhuchkaTriplesix]

Closes #1645.

Toasts (app_toast.dart): a toast faded and slid in, but when its timer ran out or its close button was pressed it was removed at once, and the toasts above it jumped to their new places.

• Closing plays the entry animation backwards and removes the toast when it ends; under reduced motion it still goes at once.
• The toast's height follows the same animation (SizeTransition), so the stack moves smoothly both when a toast arrives and when one leaves.

Welcome tour (welcome_tour_dialog.dart): every step slid in from the right, also after Back. The dialog now remembers whether the last move went forward (Next, a later dot) or back (Back, an earlier dot): the incoming step comes from that side and the outgoing one leaves on the other.

Tests: a closing toast is still on screen and partly transparent mid-exit, then gone; with reduced motion it disappears on the next frame; the tour's incoming and outgoing slides move in opposite directions, mirrored for Back. Existing toast and tour tests are unchanged.