Add cross-reference map, admin health page, passkeys and device list, settings tabs

- Reader: chapter cross-reference map (M) from IQ Bible's reference graph
- Admin: Health tab with disk use, snapshots (back up, download, restore), failure logs
- Passkey sign-in (WebAuthn), signed-in device list, sign out others, optional sign-in alerts
- Settings split into Account, Preferences, Notifications, Security, Your data tabs
- ntfy server field: catch a topic typed into the server box

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/components/ShortcutsHelp.jsx

@@ -15,6 +15,7 @@ const BY_PAGE = {
     ['G', 'Pick a book and chapter'],
     ['B', 'Bookmark the selected verse'],
     ['C', 'Compare translations'],
+    ['M', 'Map this chapter’s cross-references (needs IQ Bible)'],
     ['H', 'Your highlights'],
     [',', 'Reading settings'],
     ['F', 'Hide or show the controls'],