Restyle the UI to match lumeex's back-office look, add repo links

Reworked static/style.css to follow the same visual language as
lumeex's back office (near-black background, cyan accent, gradient
pill buttons, translucent bordered panels) instead of the previous
generic dark-teal theme. Dropzones now use a dashed border while
empty, turning solid once a file is picked. Also fixes a stale CSS
selector (.dropzone label.tag) that never matched the actual markup
(a <span>), so the SDR/HDR tag labels were unstyled before.

Adds Gitea and GitHub repo links in the footer, using the same icons
and link pattern as lumeex.

templates/index.html

@@ -59,6 +59,16 @@
       than the JPEG gain-map route.
     </p>
     <p class="version">Instagram HDR Assembler v{{ app_version }}</p>
+    <div class="repo-links">
+      <a href="https://git.djeex.fr/Djeex/insta-hdr-converter" target="_blank" rel="noopener">
+        <span class="icon"><img src="{{ url_for('static', filename='gitea.svg') }}" alt=""></span>
+        Gitea
+      </a>
+      <a href="https://github.com/Djeex/insta-hdr-converter" target="_blank" rel="noopener">
+        <span class="icon"><img src="{{ url_for('static', filename='github.svg') }}" alt=""></span>
+        GitHub
+      </a>
+    </div>
   </div>
 
   <script>