fix(mobile): stack row blocking gestures and not showing up

[mertalev]

Description

• Change viewer state to not be auto-disposed since it can be disposed before the route transition, ensuring the widgets don't start off with a null currentAsset
• Change stack row widget to not block gestures on empty space
• Center stack thumbnails and make them more compact
• Don't change thumbnail fit for active vs inactive stack items

Before:

ScreenRecording_09-12-2025.00-04-56_1.MP4

After:

ScreenRecording_09-12-2025.00-05-27_1.MP4

Scrolling:

ScreenRecording_09-12-2025.00-37-42_1.MP4

Fixes #21523 (I think)
Fixes #21582

[shenlong-tanwen]

Great work, I found some issues with my local testing:

1. If you select a stacked child, swipe up to show it's asset info and swipe down to dismiss the bottom sheet, the asset is reset to the stack parent, instead of staying at the selected child
2. If there is an stack child asset for which we couldn't render the thumbnails, we cannot select it to see the bottom sheet as well. We can swipe and select the siblings before and after the problematic child. It does work randomly, but most times, it doesn't

[mertalev]

I fixed the state issue, updated it to behave better when there are errors and made it display thumbhashes when it can't load the thumbnail. Also added a thin gray outline to unselected items for contrast.