Wire up Empty component into Marketplace and Dashboard empty states

[Copilot]

client/src/components/ui/empty.tsx provided a fully-implemented Empty component family (Empty, EmptyHeader, EmptyMedia, EmptyTitle, EmptyDescription, EmptyContent) that was never imported or used anywhere in the app. All empty states were hand-rolled <div> blocks with inconsistent structure.

Changes

• Marketplace.tsx – Replace ad-hoc "No stamps found" div with <Empty> using <EmptyMedia variant="icon"> + <Sparkles />, <EmptyTitle>, and <EmptyDescription>
• Dashboard.tsx – Replace both empty states (Favorites tab, Purchase History tab) with <Empty> using their respective icons (<Heart />, <ShoppingBag />), title/description sub-components, and CTA buttons inside <EmptyContent>

Example

<Empty>
  <EmptyHeader>
    <EmptyMedia variant="icon">
      <Heart />
    </EmptyMedia>
    <EmptyTitle>No favorites yet</EmptyTitle>
    <EmptyDescription>
      Start adding stamps to your favorites from the marketplace
    </EmptyDescription>
  </EmptyHeader>
  <EmptyContent>
    <Link href="/marketplace">
      <Button>Explore Marketplace</Button>
    </Link>
  </EmptyContent>
</Empty>

No changes to empty.tsx itself — the component was already complete.

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