fix(pg-manual): point PKG_URL at modern staging endpoint (#59)

Replace the legacy Radboud iHub PKG host
(https://main.postguard.ihub.ru.nl/pkg) with the modern staging
endpoint (https://pkg.staging.postguard.eu) so pg-manual matches
pg-sveltekit, pg-node, and pg-dotnet, which all default to
pkg.staging.postguard.eu. The /pkg path suffix is dropped because the
staging host serves the PKG v2 API at the root.

Closes #54

Co-authored-by: dobby-yivi-agent[bot] <275734547+dobby-yivi-agent[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: dobby-coder[bot]

pg-manual/examples/utils.js

@@ -8,7 +8,7 @@ export const KeySorts = {
     Signing: 'sign/key',
 }
 
-export const PKG_URL = 'https://main.postguard.ihub.ru.nl/pkg'
+export const PKG_URL = 'https://pkg.staging.postguard.eu'
 
 export async function fetchKey(
     sort,