feat(tour): highlight each step, and make the comparing steps try both looks

Owner feedback on the forced tour. Each step rings what it is about and,
while forced, dims the rest of the page around it. "Pick a layout" and the
Timeline step hold Continue until both options have been on screen, scroll
their subject to the top of the window and, on wide screens, move the card
to the right edge so the page has room to show the difference.

Guards for the reported snap-back and endless tour, which did not
reproduce: the effect that moves an unseen reader to Relaxed fires at most
once per page load and never after the tab saw the tour done, and setup
saves only the prefs it changed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/DESIGN.md

@@ -634,13 +634,21 @@ legend on the same baseline as "Next: …".
 - **The tour (2026-10-18, owner).** Every reader takes it once before using the app: new readers
   straight after setup, returning readers on their first visit, and readers who saw the earlier tour
   once more. Routines → Checklist → Pick a layout → Timeline → Redeem codes → Archive → Settings.
-  - It floats over the page it describes. While forced, the page behind is `inert` under a light
-    scrim. It can be scrolled and seen, but not clicked. There is no Skip, and a reload restarts at
-    step 1.
+  - It floats over the page it describes. While forced, the page behind is `inert`. It can be
+    scrolled and seen, but not clicked. There is no Skip, and a reload restarts at step 1.
+  - Each step rings what it is about (owner, 2026-10-18: greying the page "makes sense, but it should
+    highlight the option"): the nav tab for Routines, Checklist, Archive and Settings, the Checklist
+    or the Timeline board for the two comparing steps, the codes list for codes. While forced, the
+    rest of the page is dimmed around it. The ring is a fixed layer drawn over the target's box
+    (`TourSpotlight`), not the target raised above a scrim, so no stacking context can trap it.
   - "Pick a layout" (on the Checklist) and the Timeline step each hold a two-way switch, Relaxed /
     Structured and New / Classic. The switch changes the page behind in place, and the selected one
     is kept. Readers are shown the difference rather than told about it, because most never open
-    Appearance.
+    Appearance. Continue waits until both options have been on screen (owner: it "should *require*
+    you to have tested both"); the look showing when the step opens counts as one. These two steps
+    move the card to the right edge on wide screens (≥1100px) and scroll their subject to the top of
+    the window, past the code and release banners, so the page has room to show the difference. On a
+    phone the card stays at the foot, shortened, with the options side by side.
   - Returning readers are moved to Relaxed as the tour opens. New readers keep the look they chose in
     setup.
   - Continue stays in a fixed place and Back sits beside it. Only a replay ("Replay the