fix(calendar): correct day-of-week for full-day recurring events across all timezones (#4004)

Fixes **full-day recurring events showing on wrong day** in timezones
west of UTC (reported in #4003).

**Root cause**: `moment.tz(date, eventTimezone).startOf("day")`
interprets UTC midnight as local time:
- `2025-11-03T00:00:00.000Z` in America/Chicago (UTC-6)
- → Converts to `2025-11-02 18:00:00` (6 hours back)
- → `.startOf("day")` → `2025-11-02 00:00:00` ❌ **Wrong day!**

**Impact**: The bug affects:
- All timezones west of UTC (UTC-1 through UTC-12): Americas, Pacific
- Timezones east of UTC (UTC+1 through UTC+12): Europe, Asia, Africa -
work correctly
- UTC itself - works correctly

The issue was introduced with commit c2ec6fc2 (#3976), which fixed the
time but broke the date. This PR fixes both.

| | Result | Day | Time | Notes |
|----------|--------|-----|------|-------|
| **Before c2ec6fc2** | `2025-11-03 05:00:00 Monday` | ✅ | ❌ | Wrong
time, but correct day |
| **Current (c2ec6fc2)** | `2025-11-02 00:00:00 Sunday` | ❌ (west of
UTC)<br>✅ (east of UTC) | ✅ | Wrong day - visible bug! |
| **This fix** | `2025-11-03 00:00:00 Monday` | ✅ | ✅ | Correct in all
timezones |

Note: While the old logic had incorrect timing, it produced the correct
calendar day due to how it handled UTC offsets. The current logic fixed
the timing issue but introduced the more visible calendar day bug.

### Solution

Extract UTC date components and interpret as local calendar dates:

```javascript
const utcYear = date.getUTCFullYear();
const utcMonth = date.getUTCMonth();
const utcDate = date.getUTCDate();
return moment.tz([utcYear, utcMonth, utcDate], eventTimezone);
```

### Testing

To prevent this from happening again in future refactorings, I wrote a
test for it.

```bash
npm test -- tests/unit/modules/default/calendar/calendar_fetcher_utils_spec.js
```

modules/default/calendar/calendarfetcherutils.js

@@ -82,8 +82,12 @@ const CalendarFetcherUtils = {
 		// rrule.js returns UTC dates with tzid cleared, so we interpret them in the event's original timezone
 		return dates.map((date) => {
 			if (isFullDayEvent) {
-				// For all-day events, anchor to calendar day in event's timezone
-				return moment.tz(date, eventTimezone).startOf("day");
+				// For all-day events, extract UTC date components and interpret as local calendar date
+				// This prevents timezone offsets from shifting the date to the previous/next day
+				const utcYear = date.getUTCFullYear();
+				const utcMonth = date.getUTCMonth();
+				const utcDate = date.getUTCDate();
+				return moment.tz([utcYear, utcMonth, utcDate], eventTimezone);
 			}
 			// For timed events, preserve the time in the event's original timezone
 			return moment.tz(date, "UTC").tz(eventTimezone, true);

tests/unit/modules/default/calendar/calendar_fetcher_utils_spec.js

@@ -111,5 +111,47 @@ END:VEVENT`);
 			expect(januaryFirst[0].toISOString(true)).toContain("09:00:00.000+01:00");
 			expect(julyFirst[0].toISOString(true)).toContain("09:00:00.000+02:00");
 		});
+
+		it("should return correct day-of-week for full-day recurring events across DST transitions", () => {
+			// Test case for GitHub issue #3976: recurring full-day events showing on wrong day
+			// This happens when DST transitions change the UTC offset between occurrences
+			const data = ical.parseICS(`BEGIN:VCALENDAR
+BEGIN:VEVENT
+DTSTART;VALUE=DATE:20251027
+DTEND;VALUE=DATE:20251028
+RRULE:FREQ=WEEKLY;WKST=SU;COUNT=3
+DTSTAMP:20260103T123138Z
+UID:dst-test@google.com
+SUMMARY:Weekly Monday Event
+END:VEVENT
+END:VCALENDAR`);
+
+			const event = data["dst-test@google.com"];
+
+			// Simulate calendar with timezone (e.g., from X-WR-TIMEZONE or user config)
+			// This is how MagicMirror handles full-day events from calendars with timezones
+			event.start.tz = "America/Chicago";
+
+			const pastMoment = moment("2025-10-01");
+			const futureMoment = moment("2025-11-30");
+
+			// Get moments for the recurring event
+			const moments = CalendarFetcherUtils.getMomentsFromRecurringEvent(event, pastMoment, futureMoment, 0);
+
+			// All occurrences should be on Monday (day() === 1) at midnight
+			// Oct 27, 2025 - Before DST ends
+			// Nov 3, 2025 - After DST ends (this was showing as Sunday before the fix)
+			// Nov 10, 2025 - After DST ends
+			expect(moments).toHaveLength(3);
+			expect(moments[0].day()).toBe(1); // Monday
+			expect(moments[0].format("YYYY-MM-DD")).toBe("2025-10-27");
+			expect(moments[0].hour()).toBe(0); // Midnight
+			expect(moments[1].day()).toBe(1); // Monday (not Sunday!)
+			expect(moments[1].format("YYYY-MM-DD")).toBe("2025-11-03");
+			expect(moments[1].hour()).toBe(0); // Midnight
+			expect(moments[2].day()).toBe(1); // Monday
+			expect(moments[2].format("YYYY-MM-DD")).toBe("2025-11-10");
+			expect(moments[2].hour()).toBe(0); // Midnight
+		});
 	});
 });