fix(auth): add state parameters and PKCE support

Only the OAuth2 auth strategy was using the state parameter,
which should be used as described in the RFC. The other
auth strategies such as GitHub, GitLab or Google were lacking
the state parameter.
This change adds the required state parameter as well as
enabling PKCE support on providers where it's possible.

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

public/docs/release-notes.md

@@ -16,6 +16,7 @@
 - Force kill the server after a timeout when waiting for the realtime server to close connections on shutdown
 - Secure iframes with `credentialless` and `sandbox` attributes
 - Fix regexes for `[time=...]`, `[name=...]` and `[color=...]` shortcodes in lists
+- Use `state` parameter for OAuth2 flows and PKCE where applicable
 
 ## <i class="fa fa-tag"></i> 1.10.3 <i class="fa fa-calendar-o"></i> 2025-04-09