Polish Postmark Settings admin UI

[drudge]

Summary

UI polish from end-to-end testing of v0.1.0 against a live Postmark account.

• Persistent error banners for Send test email and Test connection failures — toasts auto-dismissed too quickly to read full Postmark API errors.
• "Make active provider" button moved up next to the alert banner so the required action is immediately actionable.
• "Test connection" grouped with the picker refresh buttons under a shared divider — they're all API health checks.
• Stats widget rendering crash fixed — Block Kit's StatsBlock expects items: StatItem[], not stats: ... (the reference doc was misleading).
• Submitted → Sent to match Postmark's own dashboard vocabulary.
• Added Failed stat card and red #DC2626 chart series so Postmark API rejections are visually distinct from recipient bounces.
• Hide the Opened stat card and Opens table column when Track opens is off, so disabled metrics don't add noise.
• Sent metric now counts only status === "sent" (in-flight, or terminal when no webhook). Previous behavior conflated "in-flight" with "all attempts."

Test plan

• pnpm typecheck — clean
• pnpm test — 110/110 pass
• pnpm build — clean
• Test against live Postmark account: trigger 401 (bad token), 422 (bad sender), and 200 success — confirm error banners persist with full message
• Verify dashboard widget cards: Sent / Delivered / Bounced / Failed (red ▼) / Opened (only when Track opens is on)
• Toggle Track opens off — confirm Opens column disappears from the recent deliveries table