New online_hot_default preference (on by default, so guests and accounts
that never saved preferences get it): opening /online without a search
replaces the URL with ?tags=order:hot — e621's metatag for the order the
Hot page uses — so it is visible in the search field and shareable.
Existing searches are never touched: they live in the URL, so a refresh
or back/forward keeps them, while a fresh visit (nav pill, first time,
after a long time away) gets the hot default again. Turning the toggle
off opens Online on the site-wide newest posts as before.
The toggle sits in Account -> Browsing preferences and saves with the
rest of the settings; the backend validates the new boolean
(400 for non-boolean input).
Adds a preferences JSON field on the user plus GET/POST
/api/auth/preferences/ (merge semantics, validated keys), surfaced in
/auth/me/ and typed on the frontend.
The Account page gains a Browsing preferences card: landing page,
default rating filter, default sort, items per page and thumbnail size.
Signed-in users also sync these while browsing (the Library sidebar's
rating/sort/per-page controls and the new thumbnail slider), debounced;
on load the account's values seed the local UI state, so settings follow
the user across browsers. Guests keep the existing localStorage
behaviour. The thumbnail size drives the media grids (Library, Online,
pool detail) between 140 and 320px columns.
Verified the API against the dev server: merge keeps untouched keys,
invalid values 400, values round-trip through /auth/me/.
JakeBreath