Online defaults to order:hot, with an Account toggle

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).

frontend/src/features/account/PreferencesCard.tsx

@@ -170,6 +170,28 @@ export function PreferencesCard() {
           />
         </label>
 
+        <label className="flex items-start gap-2 text-sm text-ctp-subtext0">
+          <input
+            type="checkbox"
+            checked={values.online_hot_default ?? true}
+            onChange={(event) =>
+              setValues((current) => ({
+                ...current,
+                online_hot_default: event.target.checked,
+              }))
+            }
+            className="mt-0.5 accent-ctp-mauve"
+          />
+          <span>
+            Online starts with{" "}
+            <span className="font-mono text-ctp-blue">order:hot</span>
+            <span className="mt-0.5 block text-[11px] leading-relaxed text-ctp-overlay0">
+              Used when you open Online without a search. With it off, Online
+              opens on the site-wide newest posts.
+            </span>
+          </span>
+        </label>
+
         <div>
           <Button onClick={() => void handleSave()} disabled={saving}>
             {saving ? "Saving…" : "Save preferences"}