Improve marketplace UX: pagination scroll restore, focus rings, async button, relative time

[xqcxx]

Closes #124
Closes #126
Closes #127
Closes #130

• Adds session-stable pagination with scroll position restore on LandingPage.
• Improves keyboard-visible focus styling on creator cards via focus-within ring.
• Introduces reusable AsyncButton for pending/disabled states and uses it for buy-key.
• Adds shared relative/absolute time helper and shows relative timestamps in transaction failure details.

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