feat: add auto login to skip sign-in method chooser

If you combine OIDC with local accounts or if your OIDC provider is an
external company (Facebook, GitHub, …) it makes sense for HedgeDoc to
show a additional login modal so users know that they will leave the
page.

However, if OAuth2/OIDC is a deployment's primary sign-in method and a
local OIDC provider is being used, users expect the common SSO method
being used, and he "Choose method" modal is a redundant extra click.
Essentially, you may have to click login thrice:

- Click login -> modal opens
- Click Login with provider -> provider login opens
- Log in (unless you are logged in already)

This simple patch adds an auto-login configuration, which makes the Sign
In button link straight to the provider's OAuth2 flow, improving the UX
of tHedgeDoc's login.

Signed-off-by: Lars Kiesow <lkiesow@uos.de>

public/docs/release-notes.md

@@ -1,5 +1,11 @@
 # Release Notes
 
+## <i class="fa fa-tag"></i> 1.x.x <i class="fa fa-calendar-o"></i> UNRELEASED
+
+### Enhancements
+
+- Automatically skip the login modal if no form-based login and exactly one external login provider is configured
+
 ## <i class="fa fa-tag"></i> 1.11.1 <i class="fa fa-calendar-o"></i> 2026-07-24
 
 ### Security fixes