fix(i18n): support Arabic comma separator in date-picker weekDays and monthNames (#14663)

## Problem

The Arabic locale strings in `ar.json` use the Arabic comma `،` (U+060C)
as separator:

```json
"com.affine.calendar-date-picker.week-days": "أ،إث،ث،أر،خ،ج،س"
```

But `day-picker.tsx` splits on ASCII comma only — causing all
weekday/month names to render as a single unsplit string in Arabic
locale.

## Fix

Change `.split(',')` to `.split(/[,،]/)` in two call sites — matches
both ASCII and Arabic comma.

## Impact

One-line fix per call site. No other functionality affected. All
non-Arabic locales unchanged.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Date picker rendering updated to correctly handle both ASCII and
Arabic/Persian comma formats when determining month and weekday labels.
This fixes inconsistent header and month-name displays in locales using
different comma characters while preserving existing interactions and
behavior.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->