Fix plugin security, hooks system, and admin features

- Add plugin allowlist (enabled_plugins in config.json)
- Add enable/disable toggle in admin (separate from visibility)
- Add plugin hooks system (actions + filters with auto-registration)
- Fix autoLinkPageTitles nested <a> tag vulnerability
- Move MQTT credentials to environment variables
- Preserve current page in language switcher
- Fix ctime/birthtime for file creation date
- Deduplicate getGuidePage() CommonMark setup
- Simplify formatDisplayName() logic
- Add admin activity log to dashboard
- Add own password change with current password verification
- Apply theme header_color to admin sidebar
- Add content preview button in editor

admin/templates/pages/content-edit.php

@@ -48,6 +48,11 @@
                 <button type="submit" class="btn btn-primary">
                     <i class="bi bi-check-lg"></i> Opslaan
                 </button>
+                <?php if ($isEditable): ?>
+                <a href="/<?= $currentLang ?>/<?= htmlspecialchars(pathinfo($file, PATHINFO_DIRNAME) . '/' . pathinfo($file, PATHINFO_FILENAME)) ?>" target="_blank" class="btn btn-outline-info" title="Open in nieuw tabblad">
+                    <i class="bi bi-eye"></i> Preview
+                </a>
+                <?php endif; ?>
                 <a href="/admin/content?dir=<?= urlencode(dirname($file)) ?>" class="btn btn-outline-secondary" id="back-btn">Terug</a>
             </div>
         </form>