fix(shell): return to the list's scroll position on Back

Back from a detail page (a biomarker, a medication, a workout) opened
the list at the top. The shell scrolls inside `<main>`, which stays
mounted across routes, and the browser's own restoration only moves
the document. Nothing kept the list's offset, so `<main>` stayed where
the detail page had left it; and the one-shot skip on `popstate` was
cleared by a timer that could run before the restored page rendered,
in which case the forward-navigation reset fired anyway.

The shell now records `<main>`'s offset per address while scrolling,
reads it back on `popstate` and applies it once the page has rendered,
retrying for up to a second while a list is still shorter than the
offset. A forward navigation still starts at the top.

src/__tests__/baselines/source-function-lines.json

@@ -138,7 +138,7 @@
     "components/labs/lab-history-list.tsx :: LabHistoryList": 534,
     "components/labs/ocr-review-dialog.tsx :: OcrReviewDialog": 359,
     "components/labs/ocr-row-editor.tsx :: OcrRowEditor": 378,
-    "components/layout/auth-shell.tsx :: AuthShell": 518,
+    "components/layout/auth-shell.tsx :: AuthShell": 484,
     "components/layout/bottom-nav.tsx :: BottomNav": 262,
     "components/layout/capture-picker.tsx :: CapturePicker": 235,
     "components/layout/sidebar-nav.tsx :: SidebarNav": 356,