fix(frontend): show local date in history cards (not UTC)

HistoryDatabase.vue formatDate() derived the day from UTC
(new Date(dateStr).toISOString().slice(0, 10)) while the sibling
formatTime() uses local hours/minutes. The backend sends naive-local
timestamps (datetime.now().isoformat()), so for any non-UTC client the
two disagree by up to a day -- e.g. for the project's UTC+8 audience a
record created 00:00-08:00 local shows the previous day's date next to
the current time. Derive the date from local components so it matches
the time shown.

frontend/src/components/HistoryDatabase.vue

@@ -312,7 +312,13 @@ const formatDate = (dateStr) => {
   if (!dateStr) return ''
   try {
     const date = new Date(dateStr)
-    return date.toISOString().slice(0, 10)
+    // Use local date components (like formatTime below) so the displayed day
+    // matches the displayed time. toISOString() converts to UTC and can show
+    // the wrong day for non-UTC clients near midnight.
+    const year = date.getFullYear()
+    const month = String(date.getMonth() + 1).padStart(2, '0')
+    const day = String(date.getDate()).padStart(2, '0')
+    return `${year}-${month}-${day}`
   } catch {
     return dateStr?.slice(0, 10) || ''
   }