GET /ponds/:id/pages lists a pond's pages ordered by the pond's
persisted sidebarSort setting (alpha/created; manual arrives with #45).
The sidebar consumes it to show the page list with an active-page
highlight, an owner-only sort switch (persists via the existing
PATCH /ponds/:id), and an inline "new page" flow. The top bar gains a
pond switcher; a new /p/:pondSlug route gives it somewhere to land,
redirecting to the pond's first page once loaded. Sidebar collapse
gains a Ctrl/Cmd+\ shortcut and a slightly refined transition.
Closes#26
