ui: separate trackers from projects; keep header dropdown on top

- /trackers index, Trackers section on the homepage and in the header nav;
  /projects lists only projects (tracker URLs unchanged)
- header is its own z-50 layer and <main> is isolated, so maps, charts and
  sticky tables no longer paint over the nav dropdowns

src/lib/title.ts

@@ -17,6 +17,7 @@ export function getTitle(path: string, environment: SiteEnvironment = null): str
 	const titles: Record<string, string> = {
 		'/': 'Home',
 		'/projects': 'Projects',
+		'/trackers': 'Trackers',
 		'/blog': 'Blog',
 		'/lists': 'Lists',
 		'/login': 'Sign in',