fix: widget backgrounds on iOS

[MikitasK]

Context

#122

Changes

Voltra now adapts iOS home widget rendering to WidgetKit presentation modes so widgets degrade predictably in Tinted & Clear appearances instead of falling back to opaque white backgrounds

• - pass WidgetKit rendering context into the iOS Voltra renderer through VoltraWidgetEnvironment
• - mark the home widget container with containerBackground(.clear, for: .widget) so WidgetKit doesn't render default opaque card
• - suppress full-bleed decorative backgrounds & glass effects for reduced home widget presentations
• - fix LinearGradient rendering for widgets, including the weather example
• - adapt text color to system-readable colors in reduced widget presentations
• - update the example weather testing screen to use correct component - IosWeatherWidget

Recording

Simulator.Screen.Recording.-.iPhone.17.Pro.-.2026-03-31.at.12.51.41.mp4

Test plan

• - visual verification of iOS available widgets in Tinted, Clear & full-color appearances
• - npm run build --workspace @use-voltra/expo-plugin
• - npm run build --workspace voltra
• - npm run typecheck --workspace voltra
• - npm run lint:libOnly
• - CI=1 WATCHMAN_DISABLE=1 npm run test --workspace voltra -- --runInBand --watchman=false
• - npm run format:check