Write a check into the list instead of rebuilding it

A check asks each saved search in turn and writes back what it found, and
the list was built again from scratch on every one of those writes. So a
background check -- which has no business being visible at all beyond a
new count and a new time -- replayed the whole list's entry animation,
threw away the reader's scroll position, and took the pointer out of
whatever row it was over, several times over.

Rows are now matched by id and told what changed: the count, the name, the
line underneath, nothing else. A row is only built when it is new, gone, or
changes shape, and the entry animation belongs to a row that has arrived
rather than to every row every time. One mid-rename or mid-confirm is left
alone entirely, so a check can no longer type over the reader.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MPZFnNdHbPGDTqQUNiE4ZN

frontend/css/style.css

@@ -3279,6 +3279,11 @@ body.theme-light .saved-badge { box-shadow: 0 0 0 2px rgba(246, 241, 231, 0.95);
     align-items: center;
     gap: 4px;
     border-bottom: 1px solid var(--border);
+}
+
+/* Rows animate in when they arrive, and only then: a row already on screen
+   that a background check wrote a new count into has not arrived anywhere. */
+.saved-row.is-new {
     animation: saved-row-in 0.3s cubic-bezier(0.16, 1, 0.3, 1) both;
     animation-delay: calc(var(--i, 0) * 40ms);
 }