feat: i18n template infrastructure — render.sh reads _en vars, emits data-de/data-en

Phase 1 of i18n rollout:
- render.sh: i18n_attrs helper, reads *_en fields from site.yaml, emits
  data-de/data-en attributes on title, description, role, tagline, cta,
  tags, section titles, card titles/descriptions, bio, content
- base.html: i18n.js auto-included, title/description get i18n attrs
- All 6 templates: translatable elements get i18n attr placeholders,
  footer toggle button with machine-translation disclaimer
- ichbinotto.de pilot: added machine-translation disclaimer per m's request

Templated sites can now be translated by adding _en fields to site.yaml.

templates/product-dark.html

@@ -95,18 +95,21 @@
 {{template_body_start}}
     <div class="container">
       <div class="hero fade-in">
-        <h1>{{title}}</h1>
-        <div class="subtitle">{{tagline}}</div>
+        <h1{{title_i18n}}>{{title}}</h1>
+        <div class="subtitle"{{tagline_i18n}}>{{tagline}}</div>
       </div>
 
 {{sections_html}}
 
       <div class="cta-section fade-in stagger-4">
-        <a href="{{cta_href}}" class="cta">{{cta_text}}</a>
+        <a href="{{cta_href}}" class="cta"{{cta_i18n}}>{{cta_text}}</a>
       </div>
 
       <div class="footer fade-in stagger-5">
         &copy; {{year}} {{name}}
+        <br>
+        <button data-i18n-toggle title="Maschinell übersetzt / Machine-translated — German is the original." style="background:none;border:1px solid currentColor;color:var(--text-dimmed,#888);font-size:0.65rem;letter-spacing:0.1em;padding:4px 12px;border-radius:4px;cursor:pointer;margin-top:12px;opacity:0.6;">EN</button>
+        <br><small data-de="Maschinell übersetzt" data-en="Machine-translated" style="color:var(--text-dimmed,#888);font-size:0.6rem;opacity:0.5;">Maschinell übersetzt</small>
       </div>
     </div>
 {{template_body_end}}