fix(desktop): remove duplicate '+' button in TabBar

[NkAntony777]

Summary

• Remove the redundant '+' (new session) button from the TabBar component
• The sidebar already has a prominent 'New Session' button, making the TabBar's small '+' button unnecessary
• This simplifies the UI and eliminates user confusion about duplicate functionality

Changes

• TabBar.tsx: Removed the '+' button, its Tooltip wrapper, and the onNewTab prop
• App.tsx: Removed the onNewTab prop from TabBar usage
• styles.css: Removed the .tabbar__new and .tabbar__new:hover CSS rules
• en.ts / zh.ts: Cleaned up the unused tabBar.newSession i18n key

Related Issue

Fixes #3933

Testing

• TypeScript type check passes (npm run typecheck)
• No new type errors introduced
• The sidebar's 'New Session' button remains functional for creating new sessions