Fix SwiftPM builds on non-Apple platforms

[jaumevn]

Motivation

• Make the package build and run tests under SwiftPM on non-Apple platforms by avoiding unconditional UIKit/CoreGraphics imports and guarding Apple-only code.

Description

• Use #if canImport(AppKit) / #elseif canImport(UIKit) for platform imports and OSColor typealias in Source/UIColor+Hex.swift so the library source compiles when AppKit/UIKit are not available.
• Wrap the OSColor extension in #if canImport(AppKit) || canImport(UIKit) so those convenience initializers are only compiled on supported platforms.
• Update XColorTests/UIColorTests.swift to conditionally import AppKit/UIKit under SwiftPM and guard the UIColor/NSColor-specific test class with canImport checks.
• Update XColorTests/Utils/UIColor+Helpers.swift to use canImport guards and wrap the OSColor helper extension accordingly, and remove the unnecessary import CoreGraphics from XColorTests/XColorTests.swift which caused failures on Linux.

Testing

• Reproduced initial failures with swift test due to no such module 'UIKit' and later no such module 'CoreGraphics' before changes.
• After applying the canImport guards and removing the Linux-incompatible import, ran swift test on x86_64-unknown-linux-gnu and all tests passed with Executed 11 tests, with 0 failures.

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Codex Task