Add admin console with login, dashboard, content/config/plugin/user management

File-based admin panel accessible at /admin.php with:
- Session-based auth with bcrypt hashing and brute-force protection
- Dashboard with site statistics and quick actions
- Content manager: browse, create, edit, delete files
- Config editor with JSON validation
- Plugin overview with status indicators
- User management: add, remove, change passwords
- CSRF protection on all forms, path traversal prevention
- Updated README (NL/EN) and guides with admin documentation

admin-console/templates/pages/config.php

@@ -0,0 +1,19 @@
+<h2 class="mb-4"><i class="bi bi-sliders"></i> Configuratie</h2>
+
+<div class="card shadow-sm">
+    <div class="card-header">
+        <i class="bi bi-filetype-json"></i> config.json
+    </div>
+    <div class="card-body">
+        <form method="POST" action="admin.php?route=config">
+            <input type="hidden" name="csrf_token" value="<?= $csrf ?>">
+            <div class="mb-3">
+                <textarea name="config_content" class="form-control font-monospace" rows="25" style="font-size: 0.9rem; tab-size: 4;"><?= htmlspecialchars($siteConfig) ?></textarea>
+                <small class="form-text text-muted">Bewerk de JSON configuratie. Ongeldige JSON wordt niet opgeslagen.</small>
+            </div>
+            <button type="submit" class="btn btn-primary">
+                <i class="bi bi-check-lg"></i> Opslaan
+            </button>
+        </form>
+    </div>
+</div>