oregon-trail: reduce displayed cover image size by 25%

#21 in Riparion/riparion-retro — merged 2026-06-10

Caps the cover art at max-w-[21rem] (75% of the prior max-w-md / 28rem), so the displayed image is 25% smaller while remaining fluid (w-full) below the cap.

• cover.rs: swap max-w-md → max-w-[21rem]
• tailwind.css: recompiled to emit the new utility class
• patch bump to 0.1.6

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