fix(admin): use useRuntimeConfig instead of process.env for provider detection

[surohak]

Description

The /_studio admin route handler reads process.env directly at runtime to detect which auth providers are configured. On platforms like AWS Amplify (and similar serverless platforms), environment variables are available during nuxt build but are not forwarded to the Lambda/function runtime.

This causes a 404 with "No authentication provider found" even though:

• The env vars exist at build time
• The module correctly populates runtimeConfig.studio.auth from those vars
• All other auth handlers already use useRuntimeConfig()

Root Cause

// admin.ts — reads process.env at runtime (broken on Amplify)
const hasGithub = process.env.STUDIO_GITHUB_CLIENT_ID && 'github'

Meanwhile github.get.ts correctly uses:

const studioConfig = useRuntimeConfig(event).studio

Fix

Replace process.env.STUDIO_* reads with useRuntimeConfig(event).studio.auth.* lookups, matching the pattern already used by all other auth route handlers.

Changes

• src/module/src/runtime/server/routes/admin.ts: Use useRuntimeConfig(event).studio.auth for provider detection

Fixes #384

[vercel]

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