Add category hover navigation

[skhuang]

Summary

• Replace the desktop first-level section strip with Overview plus taxonomy category buttons
• Show each category's section options in a hover/focus dropdown, preserving nav-btn-* selectors inside the menu
• Update browser tests to navigate through the category hover menu helper

Verification

• npm run build
• npm run test:run
• npm run test:run -- src/tests/sectionTaxonomy.test.js src/tests/urlRouter.test.js
• npm run test:browser
• npx playwright test e2e/navigation-state.spec.js e2e/accessibility-navigation.spec.js e2e/mobile-explorer.spec.js
• Playwright smoke: hover Test Generation lists Symbolic/Concolic/Fuzz/Test Generation/Exploit/SBST and clicking Fuzz opens ?section=fuzz