Improve oidc only mode (#474)

## Description of the implemented changes
The changes were:
- [x] Bugfixing
- [x] New Feature
- [ ] Breaking Change
- [x] Refactoring

**OIDC-only mode improvements and UX tweaks (success toasts, unauthenticated redirect).**

## What has been changed?

### OIDC-only mode (new feature)
- **Admin settings:** "Only OIDC sign-in" is an immediate toggle at the top of the OIDC section (no save button). Enabling it also turns off "Allow direct registration". When OIDC-only is on, the registration checkbox is disabled and shows a tooltip (DaisyUI `<.tooltip>`).
- **Backend:** Password sign-in is forbidden via Ash policy (`OidcOnlyActive` check). Password registration is blocked via validation `OidcOnlyBlocksPasswordRegistration`. New plug `OidcOnlySignInRedirect`: when OIDC-only and OIDC are configured, GET `/sign-in` redirects to the OIDC flow; GET `/auth/user/password/sign_in_with_token` is rejected with redirect + flash. `AuthController.success/4` also rejects password sign-in when OIDC-only.
- **Tests:** GlobalSettingsLive (OIDC-only UI), AuthController (redirect and password sign-in rejection), User authentication (register_with_password blocked when OIDC-only).

### UX / behaviour (no new feature flag)
- **Success toasts:** Success flash messages auto-dismiss after 5 seconds via JS hook `FlashAutoDismiss` and optional `auto_clear_ms` on `<.flash>` (used for success in root layout and `flash_group`).
- **Unauthenticated users:** Redirect to sign-in without the "You don't have permission to access this page" flash; that message is only shown to logged-in users who lack access. Logic in `LiveHelpers` and `CheckPagePermission` plug; test updated accordingly.

### Other
- Layouts: comment about unprocessed join-request count no longer uses "TODO" (Credo).
- Gettext: German translation for "Home" (Startseite); POT/PO kept in sync.
- CHANGELOG: Unreleased section updated with the above.

## Definition of Done
### Code Quality
- [x] No new technical depths
- [x] Linting passed
- [x] Documentation is added where needed (module docs, comments where non-obvious)

### Accessibility
- [x] New elements are properly defined with html-tags (labels, aria-label on checkboxes)
- [x] Colour contrast follows WCAG criteria (unchanged)
- [x] Aria labels are added when needed (e.g. oidc-only and registration checkboxes)
- [x] Everything is accessible by keyboard (toggles and buttons unchanged)
- [x] Tab-Order is comprehensible
- [x] All interactive elements have a visible focus (existing patterns)

### Testing
- [x] Tests for new code are written (OIDC-only UI, auth controller, user auth; SMTP config builder and mailer)
- [x] All tests pass
- [ ] axe-core dev tools show no critical or major issues (not re-run for this PR; suggest spot-check on settings and sign-in)

## Additional Notes
- **OIDC-only:** When the `OIDC_ONLY` env var is set, the toggle is read-only and shows "(From OIDC_ONLY)". When OIDC is not configured, the toggle is disabled.
- **Invalidation:** Enabling OIDC-only sets `registration_enabled: false` in one update; disabling OIDC-only only updates `oidc_only` (registration left as-is).
- **Review focus:** Plug order in router (OidcOnlySignInRedirect), policy/validation order in User, and that all OIDC-only paths (form, plug, controller) stay consistent.

Reviewed-on: #474
Co-authored-by: Simon <s.thiessen@local-it.org>
Co-committed-by: Simon <s.thiessen@local-it.org>

lib/mv_web/plugs/check_page_permission.ex

@@ -54,7 +54,7 @@ defmodule MvWeb.Plugs.CheckPagePermission do
         conn
         |> fetch_session()
         |> fetch_flash()
-        |> put_flash(:error, "You don't have permission to access this page.")
+        |> maybe_put_access_denied_flash(user)
         |> redirect(to: redirect_to)
         |> halt()
       end
@@ -75,6 +75,13 @@ defmodule MvWeb.Plugs.CheckPagePermission do
 
   defp redirect_target(user), do: redirect_target_for_user(user)
 
+  # Only set "no permission" flash when user is logged in; unauthenticated users get redirect only, no flash.
+  defp maybe_put_access_denied_flash(conn, nil), do: conn
+
+  defp maybe_put_access_denied_flash(conn, _user) do
+    put_flash(conn, :error, "You don't have permission to access this page.")
+  end
+
   @doc """
   Returns true if the path is public (no auth/permission check).
   Used by LiveView hook to skip redirect on sign-in etc.

lib/mv_web/plugs/oidc_only_sign_in_redirect.ex

@@ -0,0 +1,73 @@
+defmodule MvWeb.Plugs.OidcOnlySignInRedirect do
+  @moduledoc """
+  When OIDC-only mode is active:
+  - GET /sign-in redirects to the OIDC flow when OIDC is configured (sign-in page skipped).
+  - GET /sign-in?oidc_failed=1 is not redirected, so the sign-in page is shown after an OIDC
+    failure (avoids redirect loop when the provider is down or misconfigured).
+  - GET /auth/user/password/sign_in_with_token is rejected (redirect to /sign-in with error)
+  so password sign-in cannot complete.
+  """
+  import Plug.Conn
+  import Phoenix.Controller
+
+  alias Mv.Config
+
+  def init(opts), do: opts
+
+  def call(conn, _opts) do
+    conn
+    |> maybe_redirect_sign_in_to_oidc()
+    |> maybe_reject_password_token_sign_in()
+  end
+
+  defp maybe_redirect_sign_in_to_oidc(conn) do
+    if conn.request_path != "/sign-in" or conn.method != "GET" do
+      conn
+    else
+      conn = fetch_query_params(conn)
+      maybe_redirect_sign_in_to_oidc_checked(conn)
+    end
+  end
+
+  defp maybe_redirect_sign_in_to_oidc_checked(conn) do
+    cond do
+      # Show sign-in page when returning from OIDC failure to avoid redirect loop.
+      conn.query_params["oidc_failed"] -> conn
+      Config.oidc_only?() and Config.oidc_configured?() -> redirect_and_halt(conn)
+      true -> conn
+    end
+  end
+
+  defp redirect_and_halt(conn) do
+    conn
+    |> redirect(to: "/auth/user/oidc")
+    |> halt()
+  end
+
+  defp maybe_reject_password_token_sign_in(conn) do
+    if conn.halted, do: conn, else: reject_password_token_sign_in_if_applicable(conn)
+  end
+
+  defp reject_password_token_sign_in_if_applicable(conn) do
+    path = conn.request_path
+
+    password_token_path? =
+      path =~ ~r|/auth/user/password/sign_in_with_token| and conn.method == "GET"
+
+    if password_token_path? and Config.oidc_only?() do
+      message =
+        Gettext.dgettext(
+          MvWeb.Gettext,
+          "default",
+          "Only sign-in via Single Sign-On (SSO) is allowed."
+        )
+
+      conn
+      |> put_flash(:error, message)
+      |> redirect(to: "/sign-in")
+      |> halt()
+    else
+      conn
+    end
+  end
+end