perf: declutter home page by removing duplicated sections and queries

The landing page showed the same information more than once. Drop the second
avatar grid (latest users) and its DB query, move the online users grid into
the left column, remove the register banner card and the bottom join CTA so
registration is only offered once in the hero, and show the online count a
single time in the hero badge instead of also in the stats row. The online
users query now fetches 12 rows instead of 30, saving bandwidth on every
uncached render. The avatar presentation contract test now expects one
thumbnail call site on the home page.

src/components/shared/public-avatar-contract.test.ts

@@ -10,7 +10,7 @@ const compactCallSites = new Map<string, number>([
 	["src/app/(site)/login/page.tsx", 2],
 	["src/app/(site)/me/page.tsx", 1],
 	["src/app/(site)/messages/page.tsx", 2],
-	["src/app/(site)/page.tsx", 2],
+	["src/app/(site)/page.tsx", 1],
 	["src/app/(site)/radio/leaderboard/page.tsx", 1],
 	["src/app/(site)/radio/shouts/page.tsx", 1],
 	["src/app/(site)/rankings/page.tsx", 1],