Split the view step in two and add back navigation (#163)

The default view and the month style are now a step each, and both are
built from the same live-preview-over-grouped-rows shape as the Settings
choosers they duplicate, instead of chips. The month style is asked
whatever view was picked — Month is reachable from the drawer regardless,
and making the step conditional would move the counter on the step right
before it.

Back is a button in the step's top bar and the system gesture, both
clearing the previous step's answer; the plan derives position from those
answers, so that is all the navigation there is. The calendar grant has
nothing to return to, so back stops there. The slide runs backwards when
it does.

docs/ARCHITECTURE.md

@@ -94,7 +94,9 @@ pure function of the stored answers that returns the whole flow *plus* the
 step it is on, and `RootScreen` renders that step. Two rules make it work:
 
 - The plan keeps **completed** steps, so finishing one never renumbers the
-  counter under the user.
+  counter under the user, and stepping *back* is just clearing the previous
+  step's answer — there is no separate back stack. The calendar grant is the
+  one step with nothing to return to, so back stops there.
 - The optional steps (backup, view) only exist for an install that went
   through the grant in-app — recorded by `armOnboardingWizard()`, which
   refuses to arm if the reminder step was already answered, so an existing