feat(dashboard): redesign overview page with quota usage, recent traces, system info

Add quota.usage RPC method backed by QuotaChecker.Usage() which queries
per-user request counts (hour/day/week) with resolved limits and today's
aggregate stats. Dashboard now shows summary cards, quota progress bars,
recent requests table, system health, and cron jobs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/protocol/methods.go

@@ -78,6 +78,8 @@ const (
 	MethodUsageGet     = "usage.get"
 	MethodUsageSummary = "usage.summary"
 
+	MethodQuotaUsage = "quota.usage"
+
 	MethodSend = "send"
 )