calendar: avoid ambiguous timezone inference from offsets

[RaphaelRUzan]

Summary

Avoid ambiguous regional timezone inference from offset-only RFC3339 inputs.

This changes calendar event timezone extraction to use fixed-offset Etc/GMT±N zones instead of guessing named regions like America/Phoenix or America/Los_Angeles from a numeric offset alone.

Why

Offsets like -07:00 are ambiguous at a given instant and can match multiple regional IANA zones. The previous logic returned the first matching US timezone, which could mislabel Pacific events as America/Phoenix.

Using a fixed-offset timezone is less pretty, but safer and non-deceptive when the input does not carry a true named timezone.

Changes

• remove ambiguous US region guessing from extractTimezone()
• keep UTC for zero offset
• keep Etc/GMT±N fallback for whole-hour offsets
• update tests accordingly

Tests

Ran:

go test ./internal/cmd -run 'TestExtractTimezone|TestCalendarCreateCmd_RecurringIncludesTimezone|TestCalendarUpdateCmd_RecurrenceFillsMissingTimezone'

Related

• calendar: preserve named IANA time zones (Europe/Rome) instead of Etc/GMT offsets #422
• Calendar event timezone bug - events created at wrong time #423