admin: merge status + logs into unified system.php with instant tabs

Replace the separate /admin/status.php and /admin/logs.php pages with a
single /admin/system.php page organised around a tab bar.

- system.php — top-level tab bar: 'Statut' + one tab per log file
  (nginx accès, nginx erreurs, PHP-FPM).  Switching tabs is a plain
  href (?tab=…) so no JS required for navigation; the lines-selector
  SELECT triggers a location change on 'change' for instant reload
  without a submit button.
- Status tab preserves all existing service cards, PHP runtime grid,
  and disk-usage bar from the old status.php.
- Log tabs preserve line-count selector, file metadata bar, and
  per-line colour coding from the old logs.php.
- New: copy-to-clipboard button on each log output block (Clipboard
  API with textarea execCommand fallback).
- status.php / logs.php replaced with 301 redirect stubs so existing
  bookmarks and links keep working.
- templates/admin/head.php: 'Statut' + 'Journaux' nav items replaced
  with a single 'Système' item; active state covers all three page
  names for redirect compatibility.

templates/admin/head.php

@@ -29,8 +29,7 @@
         <a href="/admin/import.php" class="admin-nav__link <?= $currentPage === 'import.php' ? 'active' : '' ?>">Importer une liste de TFE</a>
         <a href="/admin/pages.php" class="admin-nav__link <?= in_array($currentPage, ['pages.php','pages-edit.php']) ? 'active' : '' ?>">Pages statiques</a>
         <a href="/admin/tags.php" class="admin-nav__link <?= $currentPage === 'tags.php' ? 'active' : '' ?>">Mots-clés</a>
-        <a href="/admin/status.php" class="admin-nav__link <?= $currentPage === 'status.php' ? 'active' : '' ?>">Statut</a>
-        <a href="/admin/logs.php" class="admin-nav__link <?= $currentPage === 'logs.php' ? 'active' : '' ?>">Journaux</a>
+        <a href="/admin/system.php" class="admin-nav__link <?= in_array($currentPage, ['system.php','status.php','logs.php']) ? 'active' : '' ?>">Système</a>
         <a href="/admin/account.php" class="admin-nav__link <?= $currentPage === 'account.php' ? 'active' : '' ?>">Compte</a>
         <?php if ($thesisId && in_array($currentPage, ['edit.php', 'thanks.php'])): ?>
             <a href="/admin/edit.php?id=<?= intval($thesisId) ?>" class="admin-nav__link <?= $currentPage === 'edit.php' ? 'active' : '' ?>">Modifier</a>