fix(types): declare fontsource modules for TypeScript 6 (ts2882)

[danfitz36]

What

Resolves the ts(2882) error on the fontsource side-effect import in src/components/CustomStyles.astro under TypeScript 6.0 (strict on by default).

Related to #707. Third and final piece of vanilla-template TS 6 readiness, alongside #708 (permalinks.ts) and #709 (blog.ts).

Why

CustomStyles.astro loads the font via a side-effect import:

import '@fontsource-variable/inter';

Fontsource packages ship CSS only — no .d.ts. Under TS 6, a side-effect import of a module with no type declarations is an error:

src/components/CustomStyles.astro:2:8 - error ts(2882): Cannot find module or type declarations for side-effect import of '@fontsource-variable/inter'.

Change

Add ambient module declarations in src/env.d.ts:

declare module '@fontsource-variable/*';
declare module '@fontsource/*';

(Both scopes are declared since the commented-out font options in CustomStyles.astro include non-variable @fontsource/* families.) Purely a type-level shim — Vite still resolves and bundles the CSS, so build output is unchanged.

Verification

With typescript@6, npx astro check no longer reports the ts(2882) error (CustomStyles.astro is clean). eslint and prettier pass.

With #708 and #709 also applied, astro check reports 0 errors on a vanilla AstroWind install under TypeScript 6.

[prototypa]

Thanks @danfitz36