Fix preview flicker while cycling windows

[ovsw]

Summary

• keep the preview pane geometry stable while a newly selected window starts producing frames
• retain the list-only fallback when the selected window cannot provide preview content
• reset preview state whenever the switcher closes

Problem

ScreencopyView.hasContent briefly becomes false when captureSource changes during Alt+Tab cycling. Because the card dimensions depend directly on hasContent, the preview pane collapses and immediately expands for each selection, producing a visible flash.

The change preserves the established preview layout for a 300 ms transition window. A successful frame keeps the pane active; otherwise it falls back to the full-width list as before.

Validation

• reproduced with the untouched upstream plugin and documented Alt+Tab bindings
• node test_model.js
• omarchy plugin validate .
• git diff --check
• CodeRabbit CLI review: 0 findings after revision