fix(storefront): fonts must cover Mongolian vowels and stop shipping latin-ext just for the tugrik sign#288

What happened

Canvas glyph probing in production Chrome shows both brand fonts fall back to system fonts for the Mongolian-specific vowels:

• Onest Variable: Ө ө Ү ү all FALLBACK
• Unbounded Variable: Ө ө Ү ү all FALLBACK

The fontsource cyrillic subset stops at U+045F. Ө is U+04E8, Ү is U+04AE. So every word like Үнэ, Өдөр, Үзэх renders half in the brand font and half in whatever the OS falls back to, on every heading, price, and product name.

At the same time, browsers download unbounded-latin-ext-wght-normal (116KB) and onest-latin-ext-wght-normal (16KB) on every page for a single glyph: ₮ (U+20AE), which sits in the latin-ext unicode-range block.

Plan

1. Build custom woff2 subsets of both variable fonts containing the Cyrillic ranges we actually use plus the missing letters (U+04AE-AF, U+04E8-EF) and ₮ (U+20AE).
2. Point the @font-face declarations at the custom subsets with matching unicode-range.
3. Drop the latin-ext files entirely.

Acceptance criteria

• Canvas probe reports no fallback for Ө ө Ү ү in either family
• No latin-ext font files are requested on any page
• Total font transfer per page drops by at least 100KB
• Headings and body text render consistently at the same weight/style for mixed Cyrillic text