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>

lang/en.json

@@ -135,34 +135,6 @@
     }
   },
   "SCRYING_POOL": {
-    "ExportPresetsTitle": "Export Scene Presets",
-    "ExportPresetsDescription": "Download all scene presets as a JSON file that can be imported into another world.",
-    "Scene": "Scene",
-    "PresetCount": "Presets",
-    "Filename": "Filename",
-    "Export": "Export",
-    "Cancel": "Cancel",
-    "Exporting": "Exporting…",
-    "ExportSuccess": "Scene presets exported successfully.",
-    "ExportFailed": "Failed to export presets",
-    "ImportPresetsTitle": "Import Scene Presets",
-    "ImportPresetsDescription": "Upload a JSON file containing scene presets to add to this scene.",
-    "SelectFile": "Select File",
-    "ChooseFile": "Choose a JSON file…",
-    "ImportMode": "Import Mode",
-    "ImportModeMerge": "Merge",
-    "ImportModeReplace": "Replace",
-    "ImportModeMergeHint": "Add new presets, skip duplicates",
-    "ImportModeReplaceHint": "Delete all existing presets and import new ones",
-    "PreviewTitle": "Preview",
-    "PresetExistsWillBeSkipped": "Already exists - will be skipped",
-    "Import": "Import",
-    "ConfirmReplace": "Replace All",
-    "Importing": "Importing…",
-    "ImportFailed": "Failed to import presets",
-    "SelectFileFirst": "Please select a file first",
-    "ExistingPresetsWarning": "This scene has {existingPresetCount} existing preset(s).",
-    "ReplaceConfirmation": "This will delete all {existingPresetCount} existing preset(s) and replace them with the imported ones. This cannot be undone.",
     "UnknownScene": "Unknown Scene",
     "firstBadgeEncounter": "First Badge Encounter"
   }