fix(dashboard): honor fixed-offset event timezones

[pi0n00r]

Summary

• normalize valid UTC+/-HH:MM and UTC+/-HHMM TZID values before the dashboard widget compares or formats event start times
• preserve the event wall clock while attaching its explicit fixed offset
• leave UTC, IANA TZIDs, floating values, all-day values, and source event data unchanged

Problem

Sabre VObject falls back to PHP's default timezone when a non-IANA fixed-offset TZID such as UTC-04:00 cannot be resolved. The Calendar view can still render the wall time correctly, but CalendarWidget receives a DateTimeImmutable carrying the fallback timezone and emits the wrong sinceId and relative subtitle.

For example, DTSTART;TZID="UTC-04:00":20260728T111500 can be emitted by the dashboard as 11:15 UTC instead of 11:15 at -04:00.

Validation

• added a full widget regression covering fixed-offset normalization
• asserted the original search result and TZID remain unchanged
• PHP 8.4 syntax checks pass for the implementation and test

The change is display-only and performs no calendar write.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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