Fix code review findings for Story 3.3: Preset Import & Export

Security & Quality Improvements:
- Fix XSS vulnerabilities in PresetImportDialog, PresetExportDialog, and templates
- Add resource leak protection in downloadExportFile() with try/finally
- Fix encapsulation violation by using public API instead of _presetsCache
- Add rollback mechanism for partial failures in replace mode
- Add preset name validation (length, characters, empty check)
- Add duplicate name detection within import files
- Add file size validation (5MB limit) and type validation
- Fix event listener leaks with proper cleanup in _onRender/_onClose
- Add constructor parameter validation for all dialogs

Acceptance Criteria Compliance:
- Fix AC-2: Export filename now uses world name (via parent.name)
- Fix AC-6: Error message matches spec exactly ('Import failed: invalid JSON format')
- Fix AC-8: Merge/Replace messages match spec format

Code Quality:
- Add shared HTML escaping utilities (src/utils/html.js)
- Consolidate duplicate localization strings (removed 28 duplicates from SCRYING_POOL)
- Use SCENE_PRESET_VERSION constant instead of hardcoded 1
- Handle null options in importPresets()
- Graceful handling of skipValidation with invalid data

Test Results: 679 passed, 3 failed (pre-existing in DirectorsBoard)

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/utils/html.js

@@ -0,0 +1,47 @@
+/**
+ * HTML escaping utilities for safe DOM manipulation.
+ *
+ * Import rule: may only import from src/contracts/.
+ * Constructors are side-effect free.
+ *
+ * @module utils/html
+ */
+
+/**
+ * Escapes HTML special characters to prevent XSS injection.
+ *
+ * @param {string} str - String to escape.
+ * @returns {string} HTML-escaped string safe for innerHTML.
+ */
+export function escapeHtml(str) {
+  if (str === null || str === undefined) return '';
+  return String(str).replace(/[&<>"']/g, (c) => ({
+    '&': '&amp;',
+    '<': '&lt;',
+    '>': '&gt;',
+    '"': '&quot;',
+    "'": '&#39;',
+  }[c]));
+}
+
+/**
+ * Safely sets innerHTML with escaped content.
+ *
+ * @param {HTMLElement} element - Target DOM element.
+ * @param {string} html - HTML content to set (will be escaped).
+ * @returns {void}
+ */
+export function safeInnerHTML(element, html) {
+  if (!element) return;
+  element.innerHTML = escapeHtml(html);
+}
+
+/**
+ * Creates a text node with escaped content.
+ *
+ * @param {string} text - Text content.
+ * @returns {Text} Text node.
+ */
+export function safeText(text) {
+  return document.createTextNode(escapeHtml(text));
+}