Fix admin CSS not loading and quirks mode issues

Fixed multiple issues in admin panel:

1. CSS path: modern-normalize.css → modern-normalize.min.css
   (File is actually named .min.css)

2. Icon path: assets/icon.svg → /assets/admin_favicon.svg
   (Was relative, now absolute; correct filename)

3. Navigation: /admin/list.php → /admin/
   (list.php was renamed to index.php)

4. Short PHP tags: <? → <?php
   (Better compatibility, some servers don't enable short_open_tag)

5. Quirks mode warning was due to CSS not loading, not DOCTYPE
   (DOCTYPE was already present)

Files modified:
- public/admin/inc/head.php (main fixes)
- public/admin/index.php (short tags)
- public/admin/add.php (short tags)
- public/admin/import.php (short tags)

Need to redeploy for production: just deploy

public/assets/admin_favicon.svg

@@ -0,0 +1,65 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   width="24"
+   height="24"
+   viewBox="0 0 24 24"
+   fill="none"
+   stroke="currentColor"
+   stroke-width="2"
+   stroke-linecap="round"
+   stroke-linejoin="round"
+   class="lucide lucide-archive-restore"
+   version="1.1"
+   id="svg12"
+   sodipodi:docname="icon.svg"
+   inkscape:version="1.2.2 (b0a8486541, 2022-12-01)"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg">
+  <defs
+     id="defs16" />
+  <sodipodi:namedview
+     id="namedview14"
+     pagecolor="#ffffff"
+     bordercolor="#000000"
+     borderopacity="0.25"
+     inkscape:showpageshadow="2"
+     inkscape:pageopacity="0.0"
+     inkscape:pagecheckerboard="0"
+     inkscape:deskcolor="#d1d1d1"
+     showgrid="false"
+     inkscape:zoom="24.719275"
+     inkscape:cx="5.2185997"
+     inkscape:cy="13.713995"
+     inkscape:window-width="1920"
+     inkscape:window-height="1011"
+     inkscape:window-x="0"
+     inkscape:window-y="32"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg12" />
+  <rect
+     width="20"
+     height="5"
+     x="2"
+     y="4"
+     rx="2"
+     id="rect2"
+     style="stroke:#c104fc;stroke-opacity:1" />
+  <path
+     d="M12 13v7"
+     id="path4"
+     style="stroke:#c104fc;stroke-opacity:1" />
+  <path
+     d="m9 16 3-3 3 3"
+     id="path6"
+     style="stroke:#c104fc;stroke-opacity:1" />
+  <path
+     d="M4 9v9a2 2 0 0 0 2 2h2"
+     id="path8"
+     style="stroke:#c104fc;stroke-opacity:1" />
+  <path
+     d="M20 9v9a2 2 0 0 1-2 2h-2"
+     id="path10"
+     style="stroke:#c104fc;stroke-opacity:1" />
+</svg>