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

assets/main.css

@@ -0,0 +1,101 @@
+body {
+  margin: 0;
+  height: 100vh;
+  overflow: hidden;
+  display: flex;
+  flex-direction: column;
+}
+
+header {
+  height: 20vh;
+}
+main {
+  height: 60vh;
+}
+footer {
+  height: 20vh;
+}
+
+body {
+  display: flex;
+  flex-direction: column;
+}
+
+header, main, footer {
+  padding: 1rem;
+  margin: 0;
+  border-radius: 40px;
+}
+
+header {
+  font-family: "police1";
+  background: #9557b5ff;
+  color: white;
+  font-size: 2rem;
+  display: flex;
+  gap: 6%;
+  padding: 1rem 4rem;
+  .title {
+    color: white;
+  }
+}
+
+header section p:not(:first-child) {
+  font-size: 14px;
+}
+
+header .title, header section, header nav {
+  text-decoration: none;
+  outline: none;
+  font-size: 18px;
+  text-decoration: none;
+  line-height: 2.5rem;
+}
+main {
+  height: 60vh;
+
+  display: grid;
+  grid-template-rows: repeat(2, minmax(0, 1fr));
+  grid-auto-flow: column;
+
+  /* critical: force column width so new columns form */
+  grid-auto-columns: 260px;
+
+  gap: 1rem;
+  padding: 1rem;
+  box-sizing: border-box;
+
+  overflow-x: auto;
+  overflow-y: hidden;
+}
+.card {
+  background: #eee;
+  border-radius: 10px;
+  padding: 1rem;
+}
+
+main {
+  background: #3c856bff;
+}
+
+footer {
+  background: #222222ff;
+}
+
+/* .card { */
+/* width: 20%; */
+/* border: 1px solid white; */
+/* color: white; */
+/* margin: 1ch; */
+/* padding: 2ch; */
+/* } */
+main {
+  scroll-snap-type: x mandatory;
+}
+
+.card {
+  scroll-snap-align: start;
+}
+.item {
+  width: 50%;
+}