feat(phase-6): rich super admin dashboard UI
Full DashboardPage rewrite with:
- 5-KPI row (total/active schools, expiring licenses, open tickets, SMS today)
- 30-day SMS volume bar chart (reuses SmsPage chart pattern)
- Revenue snapshot: MRR, paid this month, outstanding + overdue invoices
- School health table: status, tier, credit balance with LOW badge, license
expiry countdown, last-seen relative time — sortable + search + status filter
- Quick actions panel with open-ticket badge counter
- Mini stats: SMS queue depth, suspended school count
- Refresh button with last-updated timestamp
- api.ts: getDashboardSchoolHealth + getDashboardRevenue
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