[6.x] Fix missing preset tab container border under "New View" button

[jackmcdade]

Before:

After:

And on Hover it removes the border with a nice clean gap.

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Note

Improves visual consistency of the Presets tab header.

• Removes border-... styling from TabList and adjusts spacing
• Wraps the "New View" Button in a bordered container (border-b, dark mode variant, slight negative top offset) that hides on hover for a clean gap
• No logic changes; only template/class tweaks in Presets.vue

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