refactor(hig): honor Reduce Transparency and Increase Contrast for materials

[datlechin]

Summary

• New TablePro/Theme/MaterialAccessibility.swift (~75 LoC) with a themeMaterial(_:_:) view modifier that reads accessibilityReduceTransparency and colorSchemeContrast from the environment
• Apply at 7 SwiftUI material sites: Welcome sidebar, settings banners, ER diagram toolbar, JSON editor inline buttons, Pro feature scrim
• When either accessibility setting is on, the material swaps to a solid surface color picked per role (controlBackgroundColor / windowBackgroundColor / scrim variant)

Stacked on #1129. Review that one first.

Test plan

• System Settings → Accessibility → Display → "Reduce transparency" ON: Welcome left panel, settings banners, ER toolbar, JSON editor controls, Pro paywall scrim all flip from blurred to solid
• System Settings → Accessibility → Display → "Increase contrast" ON (transparency off): same surfaces flip to solid
• Both off: blur restored
• Dark mode + Reduce transparency: solid surfaces use dark-appropriate colors

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