fix(code): respect ASCII mode in splash border (#5923)

The startup splash banner now uses an ASCII-safe border when ASCII
charset mode is active.

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The splash banner kept Textual's rounded Unicode border even after its
content glyphs switched to ASCII. Apply the same charset-aware border
selection already used by the chat input and cover it with a focused
mounted-widget regression test.

Made by [Open
SWE](https://openswe.vercel.app/agents/7e5df730-093d-507b-b255-30a35d856a55)

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Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>