Make the whole iOS button group segment tappable

[vmitchell85]

On iOS, an unselected native:button-group segment only responds to taps on its label text. Tapping the empty space either side of the label does nothing, so short labels like S, M, L have a very small target. The selected segment works across its whole frame.

Each segment is a .plain style Button whose label has Color.clear as its background when unselected. A plain button only hit-tests drawn content, so the clear background doesn't count and only the text receives the tap. The selected segment paints theme.primary, which is why it behaves.

This adds .contentShape(Rectangle()) to the segment label so the whole segment is the tap target. The tab row, chip, and accordion renderers already do the same. Nothing changes visually or on the PHP side, and Android is unaffected because it uses Material's segmented button row.

#123 replaces this renderer with UISegmentedControl, which would also fix the tap area. This is the minimal fix in case that change needs longer to land.

To reproduce, tap just inside the left or right edge of an unselected segment:

<native:button-group class="w-full" :options="['S', 'M', 'L']" :value="$size" @change="pickSize" />

public int $size = 1;

public function pickSize(int $index): void
{
    $this->size = $index;
}

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