Fit version selector to selected version - #494

The app file browser currently gives every version selector a fixed width, which clips longer prerelease versions and leaves unnecessary space around short versions.

• Size supported browsers to the currently displayed option with field-sizing: content
• Keep the existing 7rem width as a fallback for older browsers
• Bound the responsive width between 5rem and the smaller of 18rem or 45vw
• Let the header control shrink-wrap the selector instead of reserving a fixed-width column

/* Before */
.version-selector { width: 7rem; }

/* After, in supporting browsers */
.version-selector {
  field-sizing: content;
  min-width: 5rem;
  max-width: min(18rem, 45vw);
}