fix(server): auto-add localhost trusted origin in dev

[onmax]

Summary

• add dev-only localhost trusted-origin augmentation when runtimeConfig.public.siteUrl is explicitly set
• detect localhost origin from Nitro dev origin (protocol + port), fallback to http://localhost:3000
• preserve trustedOrigins semantics for arrays and functions (append + dedupe, filter nullable values)
• add focused unit + e2e regression coverage for issue fix: auto-add localhost to trustedOrigins in dev mode #126

Tests

• pnpm vitest run test/get-base-url.test.ts
• pnpm vitest run test/dev-trusted-origins.test.ts
• pnpm vitest run test/server-auth-base-url-cache.test.ts
• pnpm eslint src/runtime/server/utils/auth.ts test/get-base-url.test.ts test/dev-trusted-origins.test.ts

Closes #126

[pkg-pr-new]

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npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@128

commit: 7fa52a3