Align notifications filter sheet styling with timeline content filter

[Dimillian]

Motivation

• Make the Notifications filter sheet visually match the Timeline content filter sheet so the background is glass/transparent and the sheet feels consistent.
• Replace the leading close icon with a trailing Done button to match the Timeline sheet UX.

Description

• Replaced the leading CloseToolbarItem with an explicit trailing Done button that calls dismiss() in the sheet toolbar.
• Removed usage of Theme and the opaque row background (.listRowBackground(theme.primaryBackgroundColor.opacity(0.3))) and the grouped Form style to allow the transparent/glass appearance.
• Changed presentation detents from .presentationDetents([.height(500)]) to .presentationDetents([.medium]) and removed .presentationBackground(.thinMaterial) so the system default glass presentation is used.
• Cleaned up an unused DesignSystem import that was no longer required by the view.

Testing

• Attempted an iOS build with xcodebuild -project IceCubesApp.xcodeproj -scheme IceCubesApp -destination 'platform=iOS Simulator,name=iPhone 15' build, but xcodebuild is not available in this environment so the build could not be run (failed).

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Codex Task