feat: Add welcome modal with bridge integration for new users

[BWM0223]

Summary

This PR implements the welcome modal with integrated bridge instructions as requested in #13.

Features

3-Step Welcome Flow:

Step 1 - Welcome:

• Brief introduction to CAP
• Key features highlighted (100x leverage, non-custodial, low fees, CAP rewards)

Step 2 - How It Works:

• Step-by-step guide: Connect → Bridge → Deposit → Trade

Step 3 - Get Started:

• Wallet connection button (integrates with existing web3-onboard)
• Collapsible bridge section with 4 bridge options:
  • Arbitrum Bridge (Official)
  • Synapse Protocol (Fast)
  • Stargate Finance (Multi-chain)
  • Hop Protocol (Low fees)
• Link to full documentation

Technical Details

• Uses localStorage via getUserSetting/setUserSetting to track if welcome has been shown
• Modal only shows once per browser
• Matches existing design system (colors, typography, components)
• Fully responsive
• Accessible (keyboard navigation, focus management)

Files Changed

• src/components/modals/WelcomeModal.svelte - New component
• src/components/layout/Modals.svelte - Register modal
• src/App.svelte - Auto-show on first visit

Testing

• Clear localStorage and verify modal appears
• Complete flow and verify modal does not reappear
• Test wallet connection from modal
• Test all bridge links open correctly
• Verify responsive design on mobile

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I would like to claim the $500 bounty for this issue.

Closes #13