number_input: Fix 2px height growth from the whole-control frame (#2687)

Since #2674 the frame is drawn by the wrapper, which had no fixed
height, so its 1px border grew the control past a plain `Input` of the
same size. Hold `input_h` on the wrapper and let the buttons and the
`Input` fill it.

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>