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 ✅

public/assets/common.css

@@ -22,7 +22,6 @@
 /* } */
 
 body {
-  background-color: yellow;
   margin: 0;
 }
 
@@ -82,7 +81,7 @@ label[type="button"] {
   margin-top: 2rem;
   font-size: 16px;
   border-radius: 10px;
-  padding: 2ch;
+  padding: 1rem;
   margin: 1rem;
   a {
     color: black;