feat: add web terminal, improve README, fix node-pty and i18n issues

- Add web terminal view with xterm.js and node-pty WebSocket backend
- Rewrite README with badges, feature table, mobile demo video
- Add package keywords and improved description for npm/GitHub SEO
- Fix node-pty spawn-helper missing execute permission after npm install -g
- Auto-fix node-pty permissions on CLI startup
- Fix duplicate 'error' key in en.ts and zh.ts i18n files
- Remove nested NSpin in PlatformSettings (causes invisible loading spinner)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/layout/AppSidebar.vue

@@ -259,6 +259,27 @@ function handleNav(key: string) {
         <span>{{ t("sidebar.usage") }}</span>
       </button>
 
+      <button
+        class="nav-item"
+        :class="{ active: selectedKey === 'terminal' }"
+        @click="handleNav('terminal')"
+      >
+        <svg
+          width="16"
+          height="16"
+          viewBox="0 0 24 24"
+          fill="none"
+          stroke="currentColor"
+          stroke-width="1.5"
+          stroke-linecap="round"
+          stroke-linejoin="round"
+        >
+          <polyline points="4 17 10 11 4 5" />
+          <line x1="12" y1="19" x2="20" y2="19" />
+        </svg>
+        <span>{{ t("sidebar.terminal") }}</span>
+      </button>
+
       <button
         class="nav-item"
         :class="{ active: selectedKey === 'settings' }"