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

src/cache/rate_limit/f528764d624db129b32c21fbca0cb8d6.json

@@ -1 +1 @@
-{"10":1770899252,"11":1770899274,"12":1770899301}
+[1771972436,1771972448]