fix(agenda): stop yesterday's all-day event surfacing under today (#65)

The agenda window starts at the anchor's local midnight, which east of UTC
is the previous day's 22:00 UTC. All-day events live at UTC midnights with an
exclusive next-midnight end, so yesterday's all-day event (a birthday, say)
still overlaps today's window start and is returned by the provider.
groupAgendaDays then clamped its first day up to the anchor and, via a trailing
coerceAtLeast(firstDay) on the last day, pulled it onto the anchor's "today"
section — the multi-day fix only stopped the forward leak on interior days.

Drop instances whose true last day (resolved in UTC for all-day events) falls
before the anchor, or whose first day falls past the window end: they occupy no
visible day and must not be clamped onto an edge. Adds eastern-zone regression
tests for both edges.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -46,6 +46,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   headers right below it, in the widest string in the bar. A week that straddles
   two months keeps the outgoing month until it is fully gone ([#60]).
 
+### Fixed
+- An all-day event no longer shows up again the day after it happened. In time
+  zones east of UTC, an all-day event — a birthday, say — set for one day also
+  appeared under the *next* day's heading in the Agenda (and the agenda widget),
+  because all-day events are anchored to UTC midnight and the following day's
+  window reached back across that boundary and pulled the event forward onto
+  "today". Each all-day event now lists only on the day it actually falls on
+  ([#65]).
+
 ## [2.15.0] — 2026-07-15
 
 ### Added
@@ -1040,3 +1049,4 @@ automatically, with zero telemetry and no internet permission.
 [#49]: https://codeberg.org/jlmakiola/calendula/issues/49
 [#52]: https://codeberg.org/jlmakiola/calendula/issues/52
 [#60]: https://codeberg.org/jlmakiola/calendula/issues/60
+[#65]: https://codeberg.org/jlmakiola/calendula/issues/65