fix: remove mask image if the forecast container is not scrollable

[troinine]

No description provided.

[Copilot]

Pull request overview

This PR fixes an issue where the fade mask (gradient overlay) was always applied to the forecast container, even when the content wasn't scrollable. The fix adds logic to detect when the forecast container is scrollable and conditionally applies the mask CSS only in those cases.

Key changes:

• Added scrollability detection logic that compares the number of forecast items to the calculated items that fit in the viewport
• Conditionally applied the is-scrollable CSS class using Lit's classMap directive
• Restructured CSS to only apply mask styles when the container is scrollable

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/weather-forecast-card.ts	Added _isScrollable state property, imported classMap directive, implemented logic to detect scrollable state in layoutForecastItems, and conditionally applied the is-scrollable class to the forecast container
src/weather-forecast-card.css	Moved mask-related CSS properties into a new .wfc-forecast-container.is-scrollable selector, keeping pointer-events: auto on the base container class
test/weather-forecast-card.test.ts	Added comprehensive test to verify the is-scrollable class and _isScrollable property are correctly toggled based on whether content overflows the container

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