Add Mongolian (mn) language translations for admin panel

[tuvshinjargal]

Summary

• Added 32 Mongolian translation files for the Lunar admin panel
• Covers all admin areas: products, orders, customers, discounts, channels, taxes, staff, etc.

Note

Used the AI, but I reviewed the translation as a Mongolian native speaker.

The translation follows the existing pattern of other language files (tr/, fr/, es/) in the codebase.

[alecritson]

Thanks @tuvshinjargal

I understand you have used AI to generate these but are a native speaker, I have run these through another agent and they have flagged some parts of the translations, can you take a look and just clarify whether the changes proposed look correct.

1. Garbled string repeated 14× in discount.php lines 149–360:
   'label' => 'Тү exclusion' — looks like a truncated/broken edit; not valid Mongolian. Should be e.g. Хасалт.
2. "Default" → Анхны (= "first/original") — used ~12× across channel.php:20,35, currency.php:29,50, customergroup.php:17,29, language.php:17,29, taxclass.php:14,23, taxzone.php:20,47,
   relationmanagers.php:153,164. Correct is Өгөгдмөл.
3. "Placed after/before" → Дараа захирах / Өмнө захирах (= "command/order someone after/before") in order.php:60,63. Wrong verb — should be захиалсан (ordered/placed an order).
4. "Title" (honorific: Mr/Mrs) → Гарчиг (= "page heading") in address.php:11,56 and customer.php:20,41. Should be Цол.
5. "Stock" → Агуулах (= "warehouse building") in product.php:44 and productoption.php:120. Should be Нөөц.
6. "Availability" → Боломж (= "possibility/opportunity") in product.php:96 and discount.php:121. Should be Хүртээмж / Бэлэн байдал.
7. Typo Агуулахд → should be Агуулахад (missing vowel) in productvariant.php:49,58,59.
8. "Capture Payment" → Төлөлт төлөх (= "pay the payment", redundant) in order.php action.capture_payment.label and the associated confirm hint. Should use Төлбөр баталгаажуулах.
9. "Buy X Get Y" → X аваарай Y аваарай (= "get X, get Y") in discount.php form.buy_x_get_y.heading. Meaning is wrong.

Likely issues:

• "Label" → Тайлбар (= "description") in productoption.php table.label.label / form.label.label and fieldtypes.php dropdown.form.lookups.key_label. Should be Шошго.
• "Price Breaks" → Үнийн завсарлага (= "price intermission") in relationmanagers.php pricing tab. Should be Үнийн шатлал.
• "No. Children" → Хүүхдийн тоо (= "number of human children") in collection.php. Should be Дэд коллекцийн тоо to match the page conte