#48 Add + menu button to tab bar

Summary

• Adds a "+" button to the horizontal tab bar (next to split buttons) with a dropdown menu for "New Terminal ⌘T" and "New Browser ⌘⇧L"
• Uses native NSButton + NSMenu so the icon visually matches the split buttons
• Menu appears below the button instead of at click point
• Routes creation through new didRequestNewTab delegate method in Workspace

Test plan

• Click the "+" button — menu should appear below the button
• "New Terminal" and "New Browser" menu items create the correct tab type
• "+" icon style matches the split horizontal/vertical buttons

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