fix: force end layout to use dark theme

[elecmonkey]

This PR updates the built-in end layout to always render in dark mode.

Previously, the end layout only applied a black background via its own styles, but the surrounding theme context could still be in light mode. As a result, components rendered inside the end layout (such as the built-in PoweredBySlidev component using the text-main class) would pick up light-theme colors, which are not visually correct on a black background.

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commit: b1a039e