feat: add year filter to main index

- Footer now displays all available years horizontally with scroll
- Click on year filters thesis list to that year
- Active year highlighted in footer
- 'Tous' link to reset filter
- Filter info banner shows when year selected with reset button
- Pagination preserves year filter
- Styled with horizontal scroll, smooth scrollbar
- Tests passing ✅

templates/footer.php

@@ -1,11 +1,17 @@
 <!-- footer.php -->
 <footer>
-    <div>
-        <button>Année</button>
-    </div>
-    <div>
-        <button>Cursus</button>
-    </div>
+    <nav class="years-nav">
+        <div class="years-scroll">
+            <?php if (!empty($availableYears)): ?>
+                <a href="index.php" class="year-link <?= !isset($year) ? 'active' : '' ?>">Tous</a>
+                <?php foreach ($availableYears as $y): ?>
+                    <a href="index.php?year=<?= (int)$y ?>" class="year-link <?= isset($year) && $year == $y ? 'active' : '' ?>">
+                        <?= htmlspecialchars($y) ?>
+                    </a>
+                <?php endforeach; ?>
+            <?php endif; ?>
+        </div>
+    </nav>
 </footer>
 </body>