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/header.php

@@ -8,8 +8,9 @@
     <meta name="description" content="">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <title>Posterg</title>
-    <link rel="stylesheet" href="assets/normalize.css">
-    <link rel="stylesheet" href="assets/posterg.css?v=2">
+    <link rel="stylesheet" href="assets/modern-normalize.css">
+    <link rel="stylesheet" href="assets/common.css">
+    <link rel="stylesheet" href="assets/main.css">
     <?php if (getenv('PHP_ENV') === 'development' || php_sapi_name() === 'cli-server'): ?>
         <!-- Live reload for development -->
         <script src="/vendor/php-live-reload/php-live-reload/live-reload.js"></script>