feat: add Jalali (Persian/Shamsi) calendar support with Saturday as week start (#490)

* feat: add Jalali (Persian/Shamsi) calendar support with Saturday as week start

- Add jalaali-js library for Gregorian ↔ Jalali date conversion
- Create lib/jalali-utils.ts with Jalali calendar utilities
- Create hooks/use-calendar-locale.ts for unified calendar locale handling
- Expand FirstDayOfWeek type to include 6 (Saturday)
- Update all calendar views (month, week, day, mini, toolbar) to support
  Jalali calendar display and Saturday-first week ordering
- Add Jalali month names (Farvardin … Esfand) to all locale files
- Add Persian (fa) locale with full translations
- Update settings UI to include Saturday as first day of week option
- Update useFormatEventDate to show Jalali dates when locale is fa
- Auto-detect Jalali calendar when fa locale is active

The calendar system automatically switches to Jalali when the locale is
set to Persian (fa). All internal date handling remains Gregorian (ISO
8601) for JMAP protocol compatibility; Jalali conversion is purely at
the display layer.

* Add PR template for Jalali calendar feature

* chore: remove accidentally added PR template

* fix: add image_too_large key to fa locale for PR #462 compatibility

stores/settings-store.ts

@@ -48,7 +48,7 @@ export type DateFormat = 'smart' | 'relative' | 'full';
  */
 export type DateLocale = 'auto' | 'iso' | 'en-GB' | 'en-US';
 export type TimeFormat = '12h' | '24h';
-export type FirstDayOfWeek = 0 | 1; // 0 = Sunday, 1 = Monday
+export type FirstDayOfWeek = 0 | 1 | 6; // 0 = Sunday, 1 = Monday, 6 = Saturday
 export type ExternalContentPolicy = 'ask' | 'block' | 'allow';
 export type MailAttachmentAction = 'preview' | 'download';
 export type AttachmentPosition = 'beside-sender' | 'below-header';