login: discover IdP via bare oidcIssuer term (sso#1 fix)

JSS profiles declare the OIDC issuer using the bare term
`oidcIssuer` (aliased to solid:oidcIssuer in @context). My
discoverIdpFromWebId() only looked for the prefixed form
(`solid:oidcIssuer`), so the field was never found on JSS
profiles, the fallback to `new URL(webId).origin` fired, and
solid-oidc raised "Issuer mismatch" — the WebID's origin
(test.solid.social) doesn't match the openid-config issuer
(solid.social/) that test.solid.social returns from its own
.well-known/openid-configuration.

Now reads `profile.oidcIssuer` first, falling back to the
prefixed / full-URI forms for profiles emitted by other servers
that don't alias the term.

Author: melvincarvalho

login.js

@@ -110,7 +110,12 @@ async function discoverIdpFromWebId(webId) {
     });
     if (profileRes.ok) {
       const profile = await profileRes.json();
-      const issuer = profile['solid:oidcIssuer']
+      // JSS profiles declare the IdP via the bare term `oidcIssuer`
+      // (aliased to solid:oidcIssuer in @context). Try the bare form
+      // FIRST, then the prefixed / full-URI forms as fallbacks for
+      // profiles emitted by other servers.
+      const issuer = profile.oidcIssuer
+        || profile['solid:oidcIssuer']
         || profile['http://www.w3.org/ns/solid/terms#oidcIssuer']
         || profile.solid?.oidcIssuer;
       const issuerStr = typeof issuer === 'string'