Under the ISO 8601 option, dateTimeLocale is the 'sv' sync sentinel, so any
spelled-out weekday/month name rendered with currentLocale() prints in Swedish
regardless of UI language. Follow-up to #9013/#9055 covering the surfaces those
PRs didn't: spelled-out names now follow the UI language (isoTextLocale), while
numeric dates and clock times keep currentLocale().
- scheduled-list-page: locale signal -> isoTextLocale ?? currentLocale (all 6
localeDate labels are spelled-out; fixes the reproduced 'ons, 15 juli' leak)
- dialog-focus-session-edit.formatSelectedDate: weekday+month long
- habit-tracker.dateRangeLabel: month short
- scheduled-date-group.pipe: work-view group-header tooltip weekday
- worklog (formatDayStr via mapArchiveToWorklog): day-header weekday
Inlines the isoTextLocale ?? currentLocale expression (not #9055's textLocale)
so this PR shares no files with #9055 and is conflict-free / mergeable in any
order. Adds an ISO regression test to scheduled-date-group.pipe.spec and updates
mocks so the new isoTextLocale() calls resolve. Also adds docs/handover.md.
