Add the discovery data layer behind Daily Picks, Trending and Recently Viewed

[AndrewG828]

Stacked PR 4 of 15. Based on #113 — merge in order.
#111 → #112 → #113 → #114 → #115 → #116 → #117 → #118 → #119 → #120 → #121 → #122 → #123 → #124 → #125

Three new endpoints and the two view models that drive the Explore tab. No UI
consumes them yet apart from view recording.

NetworkManager gains getDailyPicks, getTrendingPosts(category:page:limit:)
and recordPostView. The last one needed a post(url:) overload because it is
the first request in the app that sends no body and decodes no response — the
server upserts one view per viewer/post/UTC-day and ignores your own listings,
so the client can fire it unconditionally and ignore failures.

ExploreViewModel is a shared singleton with a 3-minute freshness window, since
Explore is a tab root that re-appears constantly and should not refetch on every
visit. Trending is keyed by category so switching categories bypasses the cache.

RecentlyViewedViewModel keeps its own ordered id list plus a listing cache in
AppStorage, so the section renders from disk on launch and only fetches the
posts it is missing, eight at a time. ProductDetailsViewModel.setPost is what
feeds it — opening a listing caches it locally and reports the view to the
server.

Post picks up the save-count fields. The count arrives under three different
key spellings depending on endpoint, and getPostInfo returns savers as a
user array rather than a number, so displaySaveCount takes the largest of
whatever decoded. Trending payloads often omit savers entirely, hence
ensuringMinimumSaves — if the viewer has the post saved locally, the true
count is at least one. That is a lower bound, not a global total; worth
confirming against the backend before we lean on it further.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_017R8mua9xepzF3mnRETtd6W

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