Checkpoint peering identity and opt-in discovery

public/discover.html

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+<!doctype html>
+<html lang="en">
+<head>
+<meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
+<title>Discover · MangaCircus</title>
+<link rel="stylesheet" href="app.css"><link rel="icon" href="icon.svg">
+<script src="common.js" defer></script><script src="discover.js" defer></script>
+</head>
+<body><main><p class="loading">Loading…</p></main></body>
+</html>