Add adjustable width controls for Magazine and List views

[kion]

Summary

Adds user-configurable width controls to improve screen space utilization in Magazine and List views.

Why: the List view side pane and the Magazine view central pane (both containing the list of articles) were too narrow on my 4K 32" monitor screen - it was hard to read and there was way too much wasted screen estate, so I thought it'd be great to be able to resize / set width for these panels. After implementing this tweak, it became much more convenient to use List/Magazine view modes.

Changes

• Magazine view: Added width dropdown (50%-95%) in View menu, visible only when Magazine view is active. Content is centered at all width settings.
• List view: Added draggable resize handle for the article list panel (300px - 50% of window width). Content pane temporarily hidden during resize for smooth performance.
• Both settings persist across application restarts via electron-store.

Technical Details

• Magazine width applied via inline styles to ensure proper centering
• List panel width uses direct DOM manipulation during drag, only persisting to Redux/electron-store on mouse up
• Default values:
  • Magazine view width 70%
  • List panel width 350px

Testing

• Verified width persistence after app restart
• Tested resize performance with and without selected items
• Confirmed centering works at all Magazine width percentages