feat: add theme selector to unauthenticated pages

DESIGN_GUIDELINES.md

@@ -81,7 +81,7 @@ If the `<.header>` is outside the `<.form>`, the submit button must reference th
 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)
+  - **Header:** Logo + "Mitgliederverwaltung" (left) | Club name centered via absolute positioning | Language selector + theme swap (sun/moon, DaisyUI swap with rotate) (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.
@@ -98,16 +98,18 @@ Use these standard roles:
 | Role | Use | Class |
 |---|---|---|
 | Page title (H1) | main page title | `text-xl font-semibold leading-8` |
-| Subtitle | helper under title | `text-sm text-base-content/70` |
+| Subtitle | helper under title | `text-sm text-base-content/85` |
 | Section title (H2) | section headings | `text-lg font-semibold` |
-| Helper text | under inputs | `text-sm text-base-content/70` |
-| Fine print | small hints | `text-xs text-base-content/60` |
-| Empty state | no data | `text-base-content/60 italic` |
+| Helper text | under inputs | `text-sm text-base-content/85` |
+| Fine print | small hints | `text-xs text-base-content/80` |
+| Empty state | no data | `text-base-content/80 italic` |
 | Destructive text | danger | `text-error` |
 
 **MUST:** Page titles via `<.header>`.  
 **MUST:** Section titles via `<.form_section title="…">` (for forms) or a consistent section wrapper (if you introduce a `<.card>` later).
 
+**Form labels (WCAG 2.2 AA):** DaisyUI `.label` defaults to 60% opacity and fails contrast. We override it in `app.css` to 85% of `base-content` so labels stay slightly de‑emphasised vs body text but meet the 4.5:1 minimum. Use `class="label"` and `<span class="label-text">` as usual; no extra classes needed.
+
 ---
 
 ## 4) States: Loading, Empty, Error (mandatory consistency)