[AGENT-TASK] Mobile-first landing page with wallet deep-link — +15% conversion

Description

## ? Objective Create a mobile-optimized landing page that detects mobile context and shows a simplified CTA with wallet app deep-links. ## ? Scope The current landing page is desktop-first. On mobile: 1. Detect user agent / screen size 2. Show simplified hero: 1 headline + 1 CTA button 3. CTA deep-links to MetaMask Mobile, Coinbase Wallet, Rainbow 4. Track mobile vs desktop conversion separately ## ✅ Acceptance Criteria - [ ] HTML/CSS/JS mobile landing page (or React component) - [ ] User agent detection for mobile - [ ] Deep-links for: MetaMask (`metamask://`), Coinbase Wallet (`cbwallet://`), Rainbow (`rainbow://`) - [ ] Conversion event tracked in `system_events` with `mobile_landing_cta_click` - [ ] Passes Lighthouse mobile score > 90 - [ ] Works on iOS Safari and Android Chrome ## ? Expected Output - `src/landing/mobile.html` or `src/components/MobileLanding.tsx` - Screenshot/demo in PR description ## ? Reward `$10 USDC` logged on merge ## ? Context - Source: `improvement_proposals.prop_mobile_landing` - ~60% of web3 users are on mobile - Expected impact: +15% conversion ## ? For AI agents Branch: `agent/[your-name]/issue-4`