docs: skriv det, der blev bygget, og ret det, der ikke passer længere

To nye sider. Postkasser: at stjerne i Gmail eller flage i iCloud og Fastmail
bliver til opgaver, at hver postkasse hører til den, der forbandt den, at
app-kodeord er noget andet end ens rigtige, og at Office 365 ikke kan nås ad den
vej — sidstnævnte skrevet ned frem for at blive opdaget ved at prøve. Og at logge
ind: kodeord, totrinskode og passkeys, med hvorfor en passkey ikke kan laves på
en bar IP-adresse.

Konfigurationen manglede VERDANDE_SECRET_KEY, som er den, det koster mest ikke at
vide noget om: mister man nøglen, skal postkasserne forbindes forfra. Begrundelsen
står med — det er kopierne, ikke serveren, der er problemet, for de kan hentes
gennem fladen.

Og genvejsafsnittet i quick-add beskrev q, t og u, som ikke findes længere. En
dokumentation, der lover taster, der ikke gør noget, er værre end ingen: man
prøver, der sker intet, og så tror man appen er i stykker.

Bygget med --strict; alle krydshenvisninger rammer.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/signing-in.md

@@ -0,0 +1,39 @@
+# Signing in
+
+Three ways in, and you can have all of them at once.
+
+## A password
+
+What the first account is created with. Nothing is emailed, nothing is registered
+anywhere: the first person to open a fresh instance creates the administrator
+account, and after that people arrive by invite link.
+
+## A code on top of it
+
+**Settings → Account → Two-factor.** An ordinary TOTP secret — any authenticator
+app reads the QR code. Once it is on, the password alone is not enough.
+
+Recovery codes are shown once, when you turn it on. They are the way back if the
+phone is gone, and they are not shown again.
+
+## A passkey
+
+**Settings → Account → Passkeys → Add a passkey.**
+
+A key on a device instead of a secret in your head. The private half never leaves
+the device — a phone's secure element, a laptop's keychain, a hardware key — and
+the signature is bound to the address it was made for, so a convincing copy of the
+sign-in page at a different address gets nothing.
+
+Give each one a name you will recognise later. *min bærbare* is what makes a list
+of keys reviewable, and a list nobody can read is a list nobody revokes from.
+
+Passkeys need the address in `VERDANDE_BASE_URL` to be a hostname. Browsers refuse
+to make one for a bare IP address, so an instance reached at `http://192.168.1.10:8080`
+will show the passkey section as unavailable rather than failing at the moment
+somebody tries.
+
+!!! note "One is enough, two is better"
+    Lose the only device holding your only passkey and you are back to the
+    password. Keep the password, or register a second key — a phone as well as a
+    laptop.