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>

tests/e2e/extension.test.ts

@@ -1,6 +1,9 @@
 import { test, expect } from '@playwright/test';
 
-test('LexAI popup opens correctly', async ({ page }) => {
+// TODO(P6-T1): these specs hard-code a literal [EXTENSION_ID] placeholder and can
+// never pass. Skipped until a Playwright global-setup resolves the real extension
+// ID from the loaded unpacked build's service-worker URL.
+test.skip('LexAI popup opens correctly', async ({ page }) => {
   // Navigate to extension popup
   await page.goto('chrome-extension://[EXTENSION_ID]/popup/index.html');
 
@@ -11,7 +14,7 @@ test('LexAI popup opens correctly', async ({ page }) => {
   await expect(page.locator('button:has-text("Open Settings")')).toBeVisible();
 });
 
-test('Options page saves API key', async ({ page }) => {
+test.skip('Options page saves API key', async ({ page }) => {
   await page.goto('chrome-extension://[EXTENSION_ID]/options/index.html');
 
   // Fill in API key