feat: single entry point routing — convert to front controller pattern

- Create app/public/index.php as front controller (bootstrap + Dispatcher)
- Rewrite app/router.php for PHP dev server → all non-asset requests to index.php
- Update Dispatcher to render full page layouts (head+header+view+footer)
- Move public view templates into templates/public/ (home, search, tfe, about, repertoire)
- Delete dead direct-access public/*.php files (apropos, search, tfe, licence, repertoire)
- Add clean URL routes to Dispatcher (/search, /tfe, /repertoire, /apropos, /licence, /media)
- Remove .php extensions from all internal links (header, views, templates, URLs)
- Update OG tags in controllers to use clean URLs
- Update nginx posterg.conf → front-controller try_files pattern, block direct .php access
- Update header.php and search-bar.php form actions to clean URLs
- Switch AboutController nav key from 'nav' to 'currentNav' for consistency

app/templates/search-bar.php

@@ -3,7 +3,7 @@
 // $searchValue: current search query (optional)
 $_sbValue = $searchBarValue ?? $_GET['query'] ?? '';
 ?>
-<form method="GET" action="/search.php"
+<form method="GET" action="/search"
       role="search" aria-label="Recherche">
     <label for="site-search-input" class="sr-only">Recherche</label>
     <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"