fix(terminal): WS close reason 按 UTF-8 字节截断，避免超 123 字节抛错

ws 用 Buffer.byteLength 校验 close reason 的 123 字节上限（ws/lib/sender.js），
原 slice(0, 100) 按 UTF-16 code unit 截断，非 ASCII shell 名可达 316 字节
（实测 200 个 CJK 字符），ws.close() 会抛 RangeError 顶掉原始错误。
改为按字节截断且不切碎 code point，并补回归测试。

tests/smoke.spec.ts

@@ -963,6 +963,19 @@ describe('side card settings routes', () => {
     expect(wsCloseReasonOf('plain')).toBe('plain')
   })
 
+  it('caps the close reason by UTF-8 bytes so the ws 123-byte limit holds', () => {
+    // 200 CJK characters are ~600 bytes: a character-count slice would still
+    // overflow the cap `ws` enforces with Buffer.byteLength.
+    const reason = wsCloseReasonOf(new SidebarError(
+      'shell-not-found',
+      'shell executable not found',
+      400,
+      { shell: `/bin/${'终'.repeat(200)}` },
+    ))
+    expect(reason.startsWith('shell-not-found:')).toBe(true)
+    expect(Buffer.byteLength(reason)).toBeLessThanOrEqual(123)
+  })
+
   it('reads the resolved prefs and writes a patch through the seam', async () => {
     const route = mountWithSettings(createFakeSettings())
     const read = await invoke(route, 'settings.get', {})