explore: the phone chip row says it scrolls #944

Closes #942

On a phone, the kind and sort chips on /explore now fade at the right edge, so a chip cut off at the edge says there are more.

• Where the browser ties an animation to scrolling (animation-timeline), the fade lifts once the row is scrolled to its end. Elsewhere it stays.
• It is CSS only, in src/web/style/content.ts, inside the existing 600 px rule. Desktop is unchanged.
• Looks at 390 px: at the start the mask ends 32 px from the edge and "Use" fades. At the end of the scroll the mask is 100% and "Fresh" shows in full. No horizontal overflow on the page.

Follow-up from the review of #930.

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