fix: serve logo locally to fix broken image on nav/footer

External URL was unreachable from tools subdomain (CSP or cross-origin block),
causing a grey placeholder rectangle. Logo now served from assets/images/ and
brightness/invert filter removed — logo is white-on-transparent, displays
correctly on dark nav and footer without filtering.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

includes/footer.php

@@ -4,8 +4,8 @@
 
     <div>
       <picture>
-        <source srcset="https://dobetternorge.no/assets/images/brand/logo-header.webp" type="image/webp">
-        <img class="lt-footer__logo" src="https://dobetternorge.no/assets/images/brand/logo-header.png" alt="Do Better Norge" width="140" height="34" loading="lazy">
+        <source srcset="assets/images/logo-header.webp" type="image/webp">
+        <img class="lt-footer__logo" src="assets/images/logo-header.png" alt="Do Better Norge" width="140" height="34" loading="lazy">
       </picture>
       <h3 class="lt-footer__heading">Zero Storage</h3>
       <p class="lt-footer__body">Your documents are processed entirely in memory and discarded the moment your session ends. Nothing is written to disk, cached on our servers, or retained in any database. What you upload stays private&nbsp;&mdash; always.</p>