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>

wxt.config.ts

@@ -1,4 +1,5 @@
 import { defineConfig } from 'wxt';
+import pkg from './package.json';
 
 export default defineConfig({
   extensionApi: 'chrome',
@@ -6,7 +7,9 @@ export default defineConfig({
   manifest: {
     name: 'LexAI - AI Writing Assistant',
     description: 'Grammar checking and writing assistance powered by your own LLM API key',
-    version: '1.0.0',
+    // Single source of truth: manifest version always follows package.json,
+    // so a release bump is a one-file edit and versions can never drift.
+    version: pkg.version,
     icons: {
       '16': 'icon-16.png',
       '32': 'icon-32.png',