fix(shadcn): remove unused default Next.js fonts when init replaces them

[bunlongheng]

Fixes #11124

Problem

When shadcn init applies a preset with a different font (e.g. Inter), the CLI rewrites the --font-sans theme mapping in globals.css but leaves the default create-next-app Geist import, declaration, and geistSans.variable className usage in layout.tsx. The unused font is still downloaded at runtime.

Fix

transformLayoutFonts now removes a default font (Geist -> --font-geist-sans, Geist_Mono -> --font-geist-mono) when a registry font takes over the corresponding root slot (--font-sans / --font-mono):

• Removes the .variable usage from className (template literal, cn(), or bare expression)
• Removes the variable declaration
• Removes the named import from next/font/google if no longer used
• Bails out safely if the font is referenced anywhere else (e.g. geistSans.className)
• Geist_Mono is kept when only --font-sans is replaced, since --font-mono still maps to it

Testing

3 new tests covering:

• The create-next-app repro from the issue (Geist replaced by Inter)
• Sans + mono replacement (both Geist fonts removed)
• Keep-if-referenced guard (geistSans.className preserved)

All 34 tests in update-fonts.test.ts pass; no existing test expectations changed.

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