feat: Archiv-Backfill, responsive Bilder und Site-Meta

Ergänzt Backfill- und Hilfsskripte, erzeugt nach der Pixazo-Generierung
komprimierte srcset-Varianten, validiert leere Bilder und fügt Favicon,
Web Manifest sowie Open-Graph-/Twitter-Tags in die Templates ein.

Co-authored-by: Cursor <cursoragent@cursor.com>

generator/templates/startup.html.j2

@@ -7,6 +7,10 @@
   <meta name="description" content="{{ s.subtitle }}">
   <meta property="og:title" content="MOBEA – {{ s.tagline }}">
   <meta property="og:description" content="{{ s.subtitle }}">
+  {% set og_title = 'MOBEA – ' ~ s.tagline %}
+  {% set og_description = s.subtitle %}
+  {% set og_image = hero_image.og if hero_image else none %}
+  {% include '_head_common.html.j2' %}
   {% set t = theme %}
   {% if t == 'corporate' %}
   <meta name="theme-color" content="#2563eb">
@@ -859,7 +863,7 @@
         </div>
         <div class="hero-visual">
           {% if hero_image %}
-          <img src="{{ hero_image }}" alt="MOBEA – {{ s.tagline }}" width="560" height="420">
+          <img src="{{ hero_image.src }}" srcset="{{ hero_image.srcset }}" sizes="{{ hero_image.sizes }}" alt="MOBEA – {{ s.tagline }}" width="560" height="420">
           {% else %}
           <div class="hero-gradient" aria-hidden="true">✦</div>
           {% endif %}