feat: stub implementation

[Berrysoft]

Only for developers to create their own libs without default features.

[Copilot]

Pull request overview

This PR introduces stub implementations to enable developers to create custom UI libraries without requiring default platform features. The changes allow the library to compile on any platform even when no UI backend (win32, winui, gtk, qt) is selected.

Key Changes:

• Added complete stub module with implementations for all UI widgets that panic with "stub implementation" when called
• Simplified platform-specific cfg attributes by introducing cfg_aliases for win32 and winui in build.rs
• Updated CI workflows to test stub implementations and use newer macOS runners

Reviewed changes

Copilot reviewed 31 out of 31 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
winio/src/lib.rs	Updated cfg logic to use stub module when no UI backend feature is enabled
winio/build.rs	Added cfg_aliases for win32 and winui to simplify conditional compilation
winio/Cargo.toml	Added cfg_aliases build dependency and winio-ui-windows-common dependency for Windows
winio/src/widgets/window.rs	Simplified cfg attributes from all(windows, feature = "win32") to win32 alias
winio/src/widgets/label.rs	Simplified cfg attributes using the new win32 alias
winio/src/stub/mod.rs	Core stub module with Error, Result types and not_impl panic function
winio/src/stub/runtime.rs	Stub Runtime implementation with new() and block_on() methods
winio/src/stub/ui/*.rs	Complete stub implementations for all 22 UI widget types
winio-ui-windows-common/src/darkmode/mod.rs	Added #[allow(unused)] to suppress warning for unused helper function
.github/workflows/test.yml	Updated to test stub implementations and use macos-14 runners

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