Fix timezone handling for day timeline and card dates

Revert beginningOfDay() to use local timezone methods — the UTC change
from #2790 caused cards created earlier the same day to show "yesterday"
for users in UTC-negative timezones after their local 7 PM.

Add Time.zone.name to day timeline fragment cache keys so
timezone-different renders don't collide.

app/javascript/helpers/date_helpers.js

@@ -7,7 +7,7 @@ export function signedDifferenceInDays(fromDate, toDate) {
 }
 
 export function beginningOfDay(date) {
-  return new Date(Date.UTC(date.getUTCFullYear(), date.getUTCMonth(), date.getUTCDate()))
+  return new Date(date.getFullYear(), date.getMonth(), date.getDate())
 }
 
 export function secondsToDate(seconds) {