chore: remove unused deps, pin toolchain, single-source version, quarantine e2e

- Uninstall zustand, tailwindcss, autoprefixer, tweetnacl-util (all
  unused); README no longer advertises Tailwind/Zustand.
- Fix stale 'Tone' -> 'Explain' copy in Options.
- Add .nvmrc/engines/packageManager (Node 22 to match CI).
- wxt.config.ts reads manifest version from package.json so the two
  can never drift.
- Ignore *.zip build artifacts.
- Skip the two e2e specs with literal [EXTENSION_ID] placeholders
  (cannot pass; TODO references the e2e-ID-resolution task).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -2,6 +2,10 @@
   "name": "lexai",
   "version": "1.0.0",
   "description": "A Grammarly-like Chrome Extension powered by your own LLM provider and API key",
+  "engines": {
+    "node": ">=22"
+  },
+  "packageManager": "npm@11.13.0",
   "scripts": {
     "dev": "wxt",
     "build": "wxt build",
@@ -15,18 +19,14 @@
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "tweetnacl": "^1.0.3",
-    "tweetnacl-util": "^0.15.1",
-    "wxt": "^0.20.18",
-    "zustand": "^5.0.3"
+    "wxt": "^0.20.18"
   },
   "devDependencies": {
     "@playwright/test": "^1.50.1",
     "@types/chrome": "^0.1.37",
     "@types/react": "^18.3.1",
     "@types/react-dom": "^18.3.1",
-    "autoprefixer": "^10.4.20",
     "jsdom": "^28.1.0",
-    "tailwindcss": "^3.4.17",
     "typescript": "^5.7.3",
     "vitest": "^3.0.7"
   }