Address #121 review: keep sign-out reachable, dvh peek, exact height

- Fold the account menu into the editor's mobile strip. Hiding the global
  header removed the only sign-out on mobile in the editor; the strip now
  carries it, so the space win stays but sign-out is one tap away.
- EditorRail peek cap vh → dvh, matching the dvh-bounded editor column, so
  it can't overrun the visible viewport and push the mode bar off-screen.
- Mobile editor height 4rem → 3rem: with the header hidden, only <main>'s
  py-6 (3rem) is outside the editor, so 4rem left ~16px dead. Comment
  corrected.
- Trim two comments that duplicated nearby docs.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

web/src/components/layout/AppShell.tsx

@@ -43,11 +43,10 @@ export function AppShell() {
   // max-w-5xl reading measure the other pages use (#107).
   const canvasRoute = inEditor || inPublicGarden
   const showBottomNav = !!user && !canvasRoute
-  // On a phone the editor is a full-screen canvas — the same reason the bottom
-  // bar hides there. The global top bar (brand + account) is then pure chrome
-  // above the garden, so hide it too and let the editor's own strip carry a back
-  // affordance; desktop keeps the header. Only the editor, not the public view,
-  // which has no strip of its own to fall back on.
+  // On a phone the editor is a full-screen canvas, so the global top bar is pure
+  // chrome above the garden — hide it and let the editor's own strip carry the
+  // back link AND the account menu (so sign-out isn't lost). Editor only, not the
+  // public view, which has no strip of its own to fall back on.
   const hideHeaderOnMobile = inEditor
 
   const visibleSections = sections.filter((s) => !s.adminOnly || user?.isAdmin)
@@ -126,8 +125,10 @@ export function AppShell() {
 }
 
 /** Account control: a compact button that toggles a small sign-out popover. On
- *  desktop the display name shows inline; on mobile it lives inside the popover. */
-function AccountMenu({ displayName }: { displayName: string }) {
+ *  desktop the display name shows inline; on mobile it lives inside the popover.
+ *  Exported so the editor's mobile strip can carry it — the global header that
+ *  normally hosts it is hidden there (see hideHeaderOnMobile). */
+export function AccountMenu({ displayName }: { displayName: string }) {
   const logout = useLogout()
   const navigate = useNavigate()
   const [open, setOpen] = useState(false)