Add views directory and UI improvements

- Add all view templates (servers, auth, dashboard, admin, layouts, errors, terminal)
- Fix SSH double decryption bug in SSHService
- Fix router parameter type casting for strict_types
- Add missing error views (401, 403, 404)
- Add nginx manager with file editing
- Add SSL certificates management
- Add database manager with phpMyAdmin-like interface
- Add sidebar server accordion

views/errors/500.php

@@ -0,0 +1,26 @@
+<?php
+$code = $code ?? 500;
+$message = $message ?? 'Internal Server Error';
+?>
+
+<div class="error-page">
+    <div class="error-card">
+        <div class="error-code"><?= $code ?></div>
+        <div class="error-message"><?= htmlspecialchars($message) ?></div>
+        <p class="error-description">
+            <?php if ($code === 404): ?>
+                The page you are looking for could not be found.
+            <?php elseif ($code === 403): ?>
+                You do not have permission to access this resource.
+            <?php elseif ($code === 401): ?>
+                Please log in to access this page.
+            <?php else: ?>
+                An unexpected error occurred. The administrators have been notified.
+            <?php endif; ?>
+        </p>
+        <div class="error-actions">
+            <a href="/dashboard" class="btn btn-primary">Go to Dashboard</a>
+            <button class="btn btn-secondary" onclick="history.back()">Go Back</button>
+        </div>
+    </div>
+</div>