Improve release notes accessibility and replace the embedded close icon

[HituziANDO]

Problem

The iOS release-notes close button is a 32 x 32 point image-only control without an accessibility label or hint. Its icon is decoded from a large Base64 string whenever the button is created.

Scope

• Use a system button with at least a 44 x 44 point hit target.
• Provide a localized accessibility label and an appropriate trait or hint.
• Replace the embedded Base64 bitmap with SF Symbols or a packaged vector asset.
• Preserve sufficient contrast in light and dark appearances.

Acceptance criteria

• VoiceOver announces the button's purpose.
• The tappable area is at least 44 x 44 points.
• No Base64 image is decoded during button initialization.
• The icon uses template rendering and system colors.

Tests

• Add accessibility-property assertions and verify layout at supported content sizes.

Relevant files

• Framework/Sources/ReleaseNotesController.swift