Fix OIDC: enable from runtime AUTH_OIDC_* (no NEXT_PUBLIC flag) (#5)

The SSO button required NEXT_PUBLIC_AUTH_OIDC_ENABLED, which is baked
at build time and silently fails under Docker runtime env. Drive the
login button from server-side AUTH_OIDC_* instead, and document IdP
redirect URI + AUTH_URL setup.

Co-authored-by: Cursor Agent <[email protected]>

src/lib/auth/oidc.ts

@@ -0,0 +1,7 @@
+export function isOidcConfigured(): boolean {
+  return Boolean(
+    process.env.AUTH_OIDC_ISSUER?.trim() &&
+      process.env.AUTH_OIDC_CLIENT_ID?.trim() &&
+      process.env.AUTH_OIDC_CLIENT_SECRET?.trim()
+  );
+}