Add favicon and make profile picture editable per page

- Favicon: emoji-based inline SVG (🍯), the old /images/favicon/favicon.ico
  reference never resolved to an actual file.
- Profile picture (the circular image at the top) is now a per-page
  "profile_image" field editable in /admin, instead of a hardcoded
  URL in the template.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/admin.html

@@ -137,6 +137,10 @@
         <label class="checkbox">Footer-Link-Text</label>
         <input type="text" name="footer_label" value="{{ page.footer_label or '' }}" placeholder="z.B. Honey Version">
       </div>
+      <div>
+        <label class="checkbox">Profilbild-URL</label>
+        <input type="text" name="profile_image" value="{{ page.profile_image or '' }}" placeholder="https://...">
+      </div>
       <button type="submit">Header speichern</button>
     </form>