Redesign UI to match target design images

- Flat purple-gradient nav bar with POSTERG/RÉPERTOIRE/À PROPOS links
- Full-width search bar with icon, bottom-border only, below nav
- Home: white bg, media card grid (thumbnail + author/title label below)
- Répertoire: 4-column index (Années/Catégories/Étudiantes/Mots-clés)
- TFE: 2-column layout (large text left, media right)
- À Propos: 2-column, large monospace text, new apropos.php page
- Admin: dark theme (#1a1a1a), purple gradient nav, bottom-border inputs
- New shared partials: templates/nav.php, templates/search-bar.php
- Rewrote all CSS: common, main, search, tfe, apropos, admin

templates/search-bar.php

@@ -0,0 +1,19 @@
+<?php
+// search-bar.php — shared search bar partial
+// $searchValue: current search query (optional)
+$_sbValue = $searchBarValue ?? $_GET['query'] ?? '';
+?>
+<form class="site-search" method="GET" action="/search.php">
+    <svg class="site-search__icon" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"
+         fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+        <circle cx="11" cy="11" r="8"/><line x1="21" y1="21" x2="16.65" y2="16.65"/>
+    </svg>
+    <input
+        class="site-search__input"
+        type="text"
+        name="query"
+        placeholder="Recherche..."
+        value="<?= htmlspecialchars($_sbValue) ?>"
+        autocomplete="off"
+    >
+</form>