fix(auth): improve local oauth dx

[onmax]

Summary

• hydrate runtimeConfig.public.siteUrl from NUXT_PUBLIC_SITE_URL when missing, while preserving explicit runtime config precedence
• normalize loopback host variants (127.0.0.1, ::1) to localhost in dev baseURL resolution to reduce local OAuth origin/callback mismatches
• add one-time dev diagnostic logging when baseURL is inferred instead of explicitly configured
• expose effective trusted origins from resolved Better Auth context in devtools config API
• add docs for explicit local siteUrl configuration and a troubleshooting playbook for Invalid origin + provider invalid redirect_uri
• add/extend tests for runtime config hydration, loopback normalization, and devtools trusted origins visibility

Tests

• pnpm -s eslint src/module/runtime.ts src/runtime/server/utils/auth.ts src/runtime/server/api/_better-auth/config.get.ts test/get-base-url.test.ts test/runtime-config.test.ts test/module.test.ts
• pnpm -s tsc --noEmit
• pnpm -s vitest --run test/runtime-config.test.ts test/get-base-url.test.ts test/no-db.test.ts
• pnpm -s vitest --run test/module.test.ts (fails in this environment due to missing native package @libsql/linux-x64-gnu during Nitro build)

Related: #101

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@102

commit: b52d324