[Liquid glass]: Modals

[mikhailChelbaev]

• Add shared BackgroundStyle support for modal backgrounds
• Support solid, blur, and iOS 26 liquid glass backgrounds in SwiftUI modals
• Support blur and liquid glass backgrounds in UIKit modal controllers
• Forward alert background style through AlertVM into the center modal implementation
• Add demo preview controls for modal and alert background styles

[mikhailChelbaev]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep effect backgrounds transparent when tinted

When backgroundStyle is .blur or .liquidGlass and callers also set backgroundColor, this line paints the whole contentView behind the UIVisualEffectView with that color. Because the effect view is then blurring/glassing a same-sized solid parent background instead of the presenting content, the UIKit modal no longer shows the content behind it; SwiftUI instead applies the color as a tint overlay. The filled background should be limited to .solid (or otherwise moved into the effect/tint layer) so tinted blur/glass stays translucent.

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