feat: add /compare and /guides/quick-start SEO pages

- Compare page: SnapAPI vs ScreenshotOne, URLBox, ApiFlash, CaptureKit, GetScreenshot
- Quick-start guide: 5-step developer tutorial with cURL, GET, SDK examples
- Both pages: dark theme, JSON-LD, OG tags, canonical URLs, mobile responsive
- Added clean URL redirects in routing
- Updated sitemap.xml and index.html nav
- Added seo-pages.test.ts (10 tests, all passing)

public/index.html

@@ -244,6 +244,8 @@ footer{border-top:1px solid var(--border);padding:48px 24px 32px;background:var(
       <a href="#docs">API Docs</a>
       <a href="/docs" target="_blank">Swagger</a>
       <a href="/usage">Usage</a>
+      <a href="/compare">Compare</a>
+      <a href="/guides/quick-start">Quick Start</a>
       <a href="#pricing" class="btn btn-primary btn-sm nav-cta">Get API Key</a>
     </div>
     <button class="nav-mobile" onclick="document.querySelector('.nav-links').classList.toggle('show')" aria-label="Menu">☰</button>