fix(web): use i18n translation for hardcoded "Create Session" strings (#259)

Replace hardcoded English strings with translation function calls
in SpawnSession.tsx and router.tsx. Add newSession.title key for
the NewSessionPage header to match the newSession.* namespace.

web/src/lib/locales/zh-CN.ts

@@ -93,6 +93,7 @@ export default {
   'button.paste': '粘贴',
 
   // New session form
+  'newSession.title': '创建会话',
   'newSession.machine': '机器',
   'newSession.directory': '目录',
   'newSession.placeholder': '/path/to/project',