Fix two UX issues in the settings dialog:
1. Check for Updates button width stability
- Added min-w-[140px] to prevent layout shift during state changes
- Button maintains consistent width across all states:
* "Check for Updates" (idle)
* "Checking..." (with spinner)
* "Update to vX.X.X" (with download icon)
* "Installing..." (with spinner)
2. Settings dialog scrollbar behavior
- Changed overflow-y-scroll to overflow-y-auto
- Scrollbar now only appears when content actually overflows
- Prevents unnecessary scrollbar on About tab (content < 400px)
- Maintains min-h-[400px] on all tabs to prevent dialog height jump
These changes improve visual stability and reduce UI clutter while
maintaining consistent dialog dimensions during tab switching.
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