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>

TODO.md

@@ -5,9 +5,9 @@ Ideas for the app that haven't been built yet, roughly in the order I'd do them.
 ## Running it and keeping it safe
 - [ ] **Email (SMTP).** A mail setting for password resets and invite links, so they don't depend on the ntfy topic. Also lets a
       co-teacher get their invite by email. Needs a `SMTP_URL`-style setting and a fallback to today's behaviour.
-- [ ] **Passkeys and a device list.** Sign in with Face ID or a fingerprint (WebAuthn); show where you're signed in with
+- [x] **Passkeys and a device list.** Sign in with Face ID or a fingerprint (WebAuthn); show where you're signed in with
       "sign out other devices"; optionally a login alert through ntfy.
-- [ ] **Server health page for the admin.** Last backup, disk use, failed notifications, recent sign-in failures, and a
+- [x] **Server health page for the admin.** Last backup, disk use, failed notifications, recent sign-in failures, and a
       "restore a snapshot" button (snapshots already exist under `DATA_DIR/backups`).
 - [ ] **Checks on every push.** Run tests and lint on the Gitea server, plus a few real-browser tests (Playwright) for sign-in,
       creating a study and syncing; so far everything is tested in simulation.
@@ -23,7 +23,7 @@ Ideas for the app that haven't been built yet, roughly in the order I'd do them.
 ## Reading
 - [ ] **Read-aloud with highlighting.** The device's voice reads the chapter and the verse being read is highlighted; useful for
       the BSB and NET, which have no audio.
-- [ ] **Cross-reference map.** A picture of the links between chapters, from IQ Bible's reference graph (needs the IQ Bible key).
+- [x] **Cross-reference map.** A picture of the links between chapters, from IQ Bible's reference graph (needs the IQ Bible key).
 
 ## Decided against, for now
 - Scripture memory (verse practice with spaced review).