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>

priv/gettext/default.pot

@@ -3895,3 +3895,13 @@ msgstr ""
 #, elixir-autogen, elixir-format
 msgid "User %{email}"
 msgstr ""
+
+#: lib/mv_web/live/global_settings_live.ex
+#, elixir-autogen, elixir-format
+msgid "Only OIDC sign-in is active. This option is disabled."
+msgstr ""
+
+#: lib/mv_web/controllers/auth_controller.ex
+#, elixir-autogen, elixir-format
+msgid "Only sign-in via Single Sign-On (SSO) is allowed."
+msgstr ""