fix: Displaying context menu at the wrong location

[SystemKeeper]

When long-pressing at the wrong time, we will display the context-menu/preview-cell at the wrong location and even with the wrong content. That makes it problematic to execute the actions on the correct cells.

There were multiple issues:

• The context menu active guard was not complete, a reload could happen between building the context menu and showing it
• The preview cell we dequeued was never rendered/reused, so it would show the old content
• Similar to the previous issue, the cell would be displayed at the wrong location
• The background color we use for highlighting the cell was never correctly reset, which sometimes showed the selected background on the preview cell

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI