Refactor admin panel and add migration documentation

- Add comprehensive migration guides (DEPLOYMENT_MIGRATION.md, DIRECTORY_STRUCTURE.md, MIGRATION_CHECKLIST.md)
- Refactor admin panel: split add.php, create reusable header/footer
- Update styles: admin.css, common.css, main.css
- Improve public pages: index.php, memoire.php
- Reorganize database documentation into database/docs/
- Update .gitignore and justfile

This prepares for migration to public/ directory structure

inc/footer.php

@@ -1,9 +1,12 @@
 <!-- footer.php -->
-<footer class="footer">
-  <div class="content has-text-centered">
-    <p>Site fait avec ❤ en PHP, HTML, CSS fait mains.</p>
-  </div>
+<footer>
+    <div>
+        <button>Année</button>
+    </div>
+    <div>
+        <button>Cursus</button>
+    </div>
 </footer>
 </body>
-</html>
 
+</html>