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) {

app/models/user/day_timeline.rb

@@ -42,7 +42,7 @@ class User::DayTimeline
   end
 
   def cache_key
-    ActiveSupport::Cache.expand_cache_key [ user, filter, day.to_date, events ], "day-timeline"
+    ActiveSupport::Cache.expand_cache_key [ user, filter, day.to_date, Time.zone.name, events ], "day-timeline"
   end
 
   private

app/views/events/day_timeline/_columns.html.erb

@@ -1,4 +1,4 @@
-<% cache [ day_timeline.events ] do %>
+<% cache [ day_timeline.events, Time.zone.name ] do %>
   <%# Template Dependency Updated: _layout.html.erb 2026-01-26 %>
   <% if day_timeline.has_activity? %>
     <div class="events__columns">