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/content-new.php

@@ -0,0 +1,46 @@
+<div class="d-flex justify-content-between align-items-center mb-4">
+    <h2><i class="bi bi-plus-lg"></i> Nieuwe pagina</h2>
+    <a href="admin.php?route=content&dir=<?= urlencode($dir ?? '') ?>" class="btn btn-outline-secondary btn-sm">
+        <i class="bi bi-arrow-left"></i> Terug
+    </a>
+</div>
+
+<div class="card shadow-sm">
+    <div class="card-body">
+        <form method="POST" action="admin.php?route=content-new&dir=<?= urlencode($dir ?? '') ?>">
+            <input type="hidden" name="csrf_token" value="<?= $csrf ?>">
+            <div class="row mb-3">
+                <div class="col-md-8">
+                    <label for="filename" class="form-label">Bestandsnaam</label>
+                    <input type="text" class="form-control" id="filename" name="filename" placeholder="bijv. mijn-pagina" required>
+                    <small class="form-text text-muted">Extensie wordt automatisch toegevoegd.</small>
+                </div>
+                <div class="col-md-4">
+                    <label for="type" class="form-label">Type</label>
+                    <select class="form-select" id="type" name="type">
+                        <option value="md" selected>Markdown (.md)</option>
+                        <option value="php">PHP (.php)</option>
+                        <option value="html">HTML (.html)</option>
+                    </select>
+                </div>
+            </div>
+            <?php if (!empty($dir)): ?>
+                <div class="mb-3">
+                    <small class="text-muted">Map: <?= htmlspecialchars($dir) ?></small>
+                </div>
+            <?php endif; ?>
+            <div class="mb-3">
+                <label for="content" class="form-label">Inhoud</label>
+                <textarea name="content" id="content" class="form-control font-monospace" rows="20" style="font-size: 0.9rem; tab-size: 4;" placeholder="# Mijn nieuwe pagina
+
+Schrijf hier je inhoud..."></textarea>
+            </div>
+            <div class="d-flex gap-2">
+                <button type="submit" class="btn btn-primary">
+                    <i class="bi bi-check-lg"></i> Aanmaken
+                </button>
+                <a href="admin.php?route=content&dir=<?= urlencode($dir ?? '') ?>" class="btn btn-outline-secondary">Annuleren</a>
+            </div>
+        </form>
+    </div>
+</div>