Implement interactive execution plan and strict HTML validation

Features:
- **Interactive Checklist**: 'Review Changes' modal now updates in real-time as save tasks complete.
- **Signal Logging**: Backend emits [SIGNAL] logs for deletions, adoptions, uploads, and reorders.
- **UI Cleanup**: Removed redundant textual 'Execute Progress' log pane.

Build & Quality:
- **HTML Validation**: Added 	ools/validate_html.ts to build pipeline to prevent syntax errors in embedded JS.
- **Strict Build**:
pm run build now runs alidate:html first.

package.json

@@ -6,7 +6,8 @@
     "global.ts"
   ],
   "scripts": {
-    "build": "npm run lint && webpack --mode production",
+    "validate:html": "ts-node tools/validate_html.ts",
+    "build": "npm run validate:html && npm run lint && webpack --mode production",
     "lint": "eslint \"src/**/*.{ts,js,html}\"",
     "deploy": "clasp push",
     "test": "jest",
@@ -14,14 +15,17 @@
     "prepare": "husky"
   },
   "devDependencies": {
+    "@types/cheerio": "^0.22.35",
     "@types/google-apps-script": "^1.0.85",
     "@types/jest": "^30.0.0",
     "@typescript-eslint/eslint-plugin": "^7.18.0",
     "@typescript-eslint/parser": "^7.18.0",
+    "cheerio": "^1.1.2",
     "copy-webpack-plugin": "^13.0.1",
     "eslint": "^8.57.1",
     "eslint-plugin-html": "^8.1.3",
     "gas-webpack-plugin": "^2.6.0",
+    "glob": "^13.0.0",
     "graphql-tag": "^2.12.6",
     "husky": "^9.1.7",
     "jest": "^29.7.0",