Patch the day list in place instead of navigating

Kirjaa now works like the catalog: opening a day, picking a dish, saving,
deleting, cancelling and "Näytä lisää" all patch the list where it stands.
Nothing loads a page, so the scroll position never moves.

One builder serves all three paths. buildLog takes what the screen should
show — the day, an open dish, whether the entry is being changed or a delete
confirmed — and the page render, the patch and the post-write response all go
through it. After a save it is called with only the date, so the day comes
back closed rather than reopening the sides step it was just submitted from.

Links stay links and forms stay forms, with data-on:click__prevent and
data-on:submit__prevent layered over them, so it all still works with
JavaScript off. Each response patches a single element, so plain text/html is
enough here; the SSE writer is only needed by the catalog, where the list and
both forms have to move together.

The anchors added in the previous attempt are gone. They could never have
worked: the browser positions an anchor without knowing where the page was
scrolled, so it jumped regardless.

scripts/smoke.sh

@@ -140,13 +140,31 @@ check "the saved day shows what was eaten" \
 # The selected day expands inside the list rather than in a panel above it,
 # so the rows below do not shift when one is tapped.
 day=$(curl -s -u ":$pass" "http://$addr/?pvm=2026-09-05")
-check "the selected day expands in place" "$day" 'id="paiva-2026-09-05"'
+check "the selected day expands in place" "$day" 'class="open"'
 check "and stays in the list rather than being lifted out" "$day" "kirjattu"
-check "links inside it keep the anchor" "$day" "#paiva-2026-09-05"
 
-check "saving returns to the day, not the top" \
+# ---- the day list patches in place instead of navigating ----------------
+
+dayp=$(curl -s -u ":$pass" -H 'Datastar-Request: true' "http://$addr/paiva?pvm=2026-09-05")
+check "opening a day patches the list" "$dayp" 'id="paivat"'
+refute "and returns a fragment, not a page" "$dayp" "<html"
+
+check "picking a dish patches to the sides step" \
+	"$(curl -s -u ":$pass" -H 'Datastar-Request: true' \
+		"http://$addr/paiva?pvm=2026-09-05&ruoka=$ruoka")" "Tallenna"
+
+check "saving from Datastar patches back" \
+	"$(curl -s -u ":$pass" -H 'Datastar-Request: true' \
+		-d "pvm=2026-09-04&ruoka=$ruoka" "http://$addr/kirjaa")" 'id="paivat"'
+
+check "deleting from Datastar patches back" \
+	"$(curl -s -u ":$pass" -H 'Datastar-Request: true' \
+		-d "pvm=2026-09-04" "http://$addr/poista")" 'id="paivat"'
+
+# Without the header it must still redirect, for no JavaScript.
+check "a plain save still redirects to the day" \
 	"$(curl -s -o /dev/null -w '%{redirect_url}' -u ":$pass" \
-		-d "pvm=2026-09-04&ruoka=$ruoka" "http://$addr/kirjaa")" "#paiva-2026-09-04"
+		-d "pvm=2026-09-04&ruoka=$ruoka" "http://$addr/kirjaa")" "pvm=2026-09-04"
 
 # Deleting a logged meal drops the row outright, so it asks first.
 saved=$(curl -s -u ":$pass" "http://$addr/?pvm=2026-09-05&poista=1")