[frontend] Marketing static pages

[Kaylahray]

Description

Create /about, /for-builders, and /for-contributors landing sub-pages with content from the project README.

User Story

As a visitor,
I want to learn about Quid and how it works for my role,
so that I can decide whether to sign up as a creator or hunter.

Requirements and Context

• New files: src/app/about/page.tsx, src/app/for-builders/page.tsx, src/app/for-contributors/page.tsx
• Content sourced from Quid/README.md value props
• Stellar-native messaging: escrow, USDC, Soroban, reputation
• Wire hero nav links in hero-section.tsx

Suggested Implementation

// IN FILE: src/app/for-builders/page.tsx
// 1. Hero + 3-step how-it-works for founders.
// 2. Feature list: Bounty Vault, IPFS proof, asset gating, reputation.
// 3. CTA → Sign Up (link to /connect-wallet).

Acceptance Criteria

• Three static pages exist and render.
• Hero nav links route to correct pages.
• Each page has a Sign Up CTA.
• Copy matches Quid product terminology (missions, not surveys).

Submission Guidelines

• Branch: frontend/marketing-pages
• Depends on: None
• PR: feat(frontend): add about and role marketing pages