fix: obfuscate email in contact links, raise rate limits, make Libre toggleable

- about.php: use EmailObfuscator::email() for contact email link text instead of htmlspecialchars
- SearchController: raise rate limit from 30 to 300 req/min
- request-access.php: raise rate limit from 3 to 30 req/10min
- partage/index.php: raise rate limit from 5 to 50 req/10min
- contenus.php: make Libre option toggleable (remove disabled class), move to top of Degré d'ouverture, remove temporary note about next academic year

app/public/partage/index.php

@@ -471,7 +471,7 @@ function handleShareLinkSubmission(string $slug): void
     // 5 submissions per IP per 10 minutes, keyed per share link.
     $rateLimitCacheDir = STORAGE_ROOT . '/cache/rate_limit';
     $shareRateLimitId  = 'share_' . $slug . '_' . ($_SERVER['REMOTE_ADDR'] ?? 'unknown');
-    $rateLimit         = new RateLimit(5, 600, $rateLimitCacheDir);
+    $rateLimit         = new RateLimit(50, 600, $rateLimitCacheDir);
 
     if (!$rateLimit->checkKey($shareRateLimitId)) {
         $_SESSION['_flash_error'] = 'Trop de tentatives. Veuillez réessayer plus tard.';