Add mobile app CTAs to landing hero

[panxiaoqi156-ship-it]

Summary

• Update the landing hero to emphasize onchain work from web and mobile
• Add Browse tasks, iOS app, and Android app CTAs wired through siteConfig
• Keep the hero title and dashboard preview constrained on mobile to avoid horizontal overflow
• Align the page metadata with the updated landing page positioning

Refs #86 and the funded Gibwork landing page improvement task.

Validation

• npm run build
• Verified desktop layout at 1440x1100 with Chrome DevTools Protocol: clientWidth 1440, scrollWidth 1440
• Verified mobile layout at 390x844 with Chrome DevTools Protocol: clientWidth 390, scrollWidth 390

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