Fix locale font on the first-install popup

[hiltay]

What does this PR do?

The first-install popup can show missing-glyph boxes on a localized client before the options addon has initialized EllesmereUI._font. Its hard-coded Expressway fallback cannot render Chinese. Use the existing locale-aware GetFontPath() resolver so the initial setup labels are readable. The core defines this resolver before the popup file loads.

How was it tested?

• Live macOS client 12.1.0.69587, zhCN: reproduced the boxes with the upstream popup file, then verified readable Chinese after the one-line fix and a UI reload, before opening options.
• Verified the live locale and resolver use Fonts\ARKai_T.ttf; the Reload UI button still applies the existing selections.
• Lua 5.1 syntax, git diff --check, and the repository's locale-key regeneration check pass (781 keys, no generated-file changes).
• Other locales were not tested in-game; they continue using the existing shared font resolver. No translation strings are added or changed.

Screenshots

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After:

Checklist

• New settings default OFF (N/A: no new settings; fixes existing localized setup)
• Zero cost while disabled: no new events, polling, hooks, or frames
• Cheap while enabled: existing popup construction only; no new update work
• No writes onto Blizzard-owned frames; no hook or script changes
• Tested in-game on live; no version gates or pre-Midnight APIs added