fix: translation of login page

DESIGN_GUIDELINES.md

@@ -76,6 +76,21 @@ For LiveViews that render an edit or new form (e.g. member, group, role, user, c
 
 If the `<.header>` is outside the `<.form>`, the submit button must reference the form via the `form` attribute (e.g. `form="user-form"`).
 
+### 2.3 Public / unauthenticated pages (Join, Sign-in, Join Confirm)
+
+Pages that do not require authentication (e.g. `/join`, `/sign-in`, `/confirm_join/:token`) use a unified layout via the **`Layouts.public_page`** component:
+
+- **Component:** `Layouts.public_page` renders:
+  - **Header:** Logo + "Mitgliederverwaltung" (left) | Club name centered via absolute positioning | Language selector (right)
+  - Main content slot, Flash group. No sidebar, no authenticated-layout logic.
+- **Content:** DaisyUI **hero** section (`hero`, `hero-content`) for the main message or form, so all public pages share the same visual structure. The hero is constrained in width (`max-w-4xl mx-auto`) and content is left-aligned (`hero-content flex-col items-start text-left`).
+- **Locale handling:** The language selector uses `Gettext.get_locale(MvWeb.Gettext)` (backend-specific) to correctly reflect the active locale. `SignInLive` sets both `Gettext.put_locale(MvWeb.Gettext, locale)` and `Gettext.put_locale(locale)` to keep global and backend locales in sync.
+- **Translations for AshAuthentication components:** AshAuthentication’s `_gettext` mechanism translates button labels (e.g. “Sign in” → “Anmelden”, “Register” → “Registrieren”) at runtime via `gettext_fn: {MvWeb.Gettext, "auth"}`. Components that do NOT use `_gettext` (e.g. `HorizontalRule`) receive static German overrides via **`MvWeb.AuthOverridesDE`**, which is prepended to the overrides list in `SignInLive` when the locale is `"de"`.
+- **Implementation:**
+  - **Sign-in** (`SignInLive`): Uses `use Phoenix.LiveView` (not `use MvWeb, :live_view`) so AshAuthentication’s sign_in_route live_session on_mount chain is not mixed with LiveHelpers hooks. Renders `<Layouts.public_page flash={@flash}>` with the SignIn component inside a hero. Displays a locale-aware `<h1>` title (“Anmelden” / “Registrieren”) above the AshAuthentication component (the library’s Banner is hidden via `show_banner: false`).
+  - **Join** (`JoinLive`): Uses `use MvWeb, :live_view` and wraps content in `<Layouts.public_page flash={@flash}>` with a hero for the form.
+  - **Join Confirm** (controller): Uses `JoinConfirmHTML` with a template that repeats the same header markup and a hero block for the result (no component call from controller templates).
+
 ## 3) Typography (system)
 
 Use these standard roles: