perf(build): measure the homepage and fix optimizePackageImports

/ is the highest-traffic route but was missing from both the public-route
instrumentation and the JS size budgets, so a payload regression there could
never fail the build. It is now wrapped in withPublicPagePerformance and
budgeted alongside the other public routes.

optimizePackageImports listed date-fns, which is not a dependency (a no-op),
while omitting the barrel-heavy packages that are actually imported: motion,
@dnd-kit/core, @dnd-kit/sortable and @tanstack/react-virtual.

src/lib/public-page-performance.ts

@@ -4,6 +4,7 @@ import { collectPerformance } from "./performance-diagnostics";
 import { type PerformanceSample, recordPerformance } from "./performance-store";
 
 type PublicRoute =
+	| "/"
 	| "/me"
 	| "/news"
 	| "/news/[slug]"