feat: Implement Account Type Selection Screen

[Ai-chan-0411]

Closes #21

Summary

Implements the Account Type Selection screen as the first onboarding step for new users.

Changes

• src/components/account-type-card.tsx — Reusable card component with hover and selected states (border glow + background highlight)
• src/app/create-account/page.tsx — Account type selection page with two card options

Features

• Two selectable account types in card format: "Create Surveys & Get Insights" and "Take Surveys & Earn Money"
• Visual hover state with purple border glow
• Selected state with purple background glow and border
• Selection routes to /create-account/surveys or /create-account/respondent
• Responsive layout (single column on mobile, two columns on md+)
• Framer Motion entrance animations
• Uses Shadcn UI patterns and Tailwind CSS utility classes matching existing theme (primary-purple, bg-dark, etc.)
• Continue button disabled until a selection is made

/attempt 21

[Ai-chan-0411]

Closing due to no maintainer review in 48h+. Thank you for considering!