Align button areas to consistent width across SegmentedRowView rows

[timbms]

Summary

• Segmented, press-release and single-mapping button areas on the same page previously had different widths, making the list look ragged
• Introduces SegmentedButtonAreaWidthKey (PreferenceKey, max-reduce) and segmentedButtonAreaMaxWidth (@Entry environment value) to propagate the page-wide maximum button-area width
• alignSegmentedButtonAreas() on the List in SitemapPageView (and PreviewList) wires onPreferenceChange → environment
• buttonAreaWidth() on each button-area type measures natural width via a GeometryReader background and applies an exact frame(width: maxWidth, alignment: .trailing) once the page-wide max is known — all three button types (segmented, press-release, single-mapping) share the same frame width and stay trailing-aligned within it

Test plan

• Open a sitemap page with a mix of segmented, press-release and single-mapping rows — verify all button areas are the same width and flush with the trailing edge
• Verify the selection indicator animation and optimistic selection still work correctly on segmented rows
• Verify press-and-hold behaviour on press-release buttons is unaffected
• Verify single-mapping button tap behaviour is unaffected
• Check layout on pages that contain only one segmented row (no alignment effect expected)
• Check layout with very long labels to confirm the button area does not overflow its row insets