feat: Allow date format preference in user interface settings

[webalexeu]

Adds a date format dropdown in User Interface settings with four options:

• Auto: locale-native ordering (existing behavior, unchanged)
• DD/MM/YYYY: day-first ordering
• MM/DD/YYYY: month-first ordering
• YYYY-MM-DD: ISO ordering

Month and weekday names always follow the UI locale regardless of the chosen ordering. The setting is persisted to localStorage.

fixes #21910

[webalexeu]

Proposed improvement for #21910

[sourcery-ai]

Hey - I've left some high level feedback:

• In formatNumericDate, the ymd branch uses Intl.DateTimeFormat with only { month: 'numeric', day: 'numeric' }, so the year is never included; if you intend YYYY-MM-DD as per the comment, you likely need to construct that explicitly (similar to formatDayMonth) instead of relying on the en-CA locale.
• The dateFormat setting is treated as a free-form string in multiple places; consider introducing a shared union type or enum (e.g. '' | 'dmy' | 'mdy' | 'ymd') to constrain and validate values across settings.ts, units.ts, and the formatter mixin.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In `formatNumericDate`, the `ymd` branch uses `Intl.DateTimeFormat` with only `{ month: 'numeric', day: 'numeric' }`, so the year is never included; if you intend `YYYY-MM-DD` as per the comment, you likely need to construct that explicitly (similar to `formatDayMonth`) instead of relying on the en-CA locale.
- The `dateFormat` setting is treated as a free-form `string` in multiple places; consider introducing a shared union type or enum (e.g. `'' | 'dmy' | 'mdy' | 'ymd'`) to constrain and validate values across `settings.ts`, `units.ts`, and the formatter mixin.

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