Implement document tab manager and tab UI

[beNative]

Summary

• replace the single document selection state with a tab manager that tracks open documents and the active tab
• update document creation, selection, and deletion flows to keep the tab list consistent
• add a DocumentTabs component that renders closable, reorderable tabs with an overflow menu

Testing

• npm run build

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https://chatgpt.com/codex/tasks/task_e_68e1446ae13c83328c4bf8be7dffd59d