feat(adaptive): ✨ enhance adaptive transition for web and preserve custom transitions

[3d24rd0]

Description

• Preserve Explicit Route Transitions: When a route declares an explicit non-default transition (e.g., `fade`, `scale`, `none`), `Browser.defaultAdaptiveTransition` now preserves it instead of overriding it with platform defaults.
• Web & Desktop Optimization: Maps Web, Windows, and Linux to `RouteTransition.only_hero` by default, avoiding awkward lateral slide transitions on wide viewports while allowing Hero animations to shine.
• Mobile Native Parallax & Motion: Retains `RouteTransition.slide_cupertino` on iOS/macOS and `RouteTransition.shared_axis_x` on Android/Fuchsia.
• Integration Tests: Added unit and integration tests covering default adaptive resolution and explicit transition preservation across all target platforms.

Tests & Analysis

• `dart analyze`: 0 issues found.
• `flutter test`: 106/106 tests passing.