feat: Rename "Copy Tab" to "Duplicate Tab" in context menu

[Copilot]

Users requested a "Duplicate" option in the tab context menu, analogous to browser tab duplication.

Changes

• TabContextMenu.ts: Renamed copyTabEl → duplicateTabEl and updated the menu item label from "Copy Tab" → "Duplicate Tab"

The underlying behavior is unchanged — duplicating clones the tab's persisted config with a new random ID and opens it as the active tab.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Duplicate tab</issue_title>
<issue_description>It's often the case that a version of a query needs to be written in another tab. Like with browser tabs, it would be nice to have a Duplicate option in the tab menu.</issue_description>

<agent_instructions>In the context menu of the yasgui tabs, add the option to duplicate the tab</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Duplicate tab #136

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