fix(auth): passkey sign-in polish #271

Problem

• Autofill is advertised but never used. The handle input has autocomplete="username webauthn" (crates/ui/src/form.rs:637) and the conditional-ui feature is on, but mediation is None (apps/auth-service/src/passkey/mod.rs:640) and the script never asks for conditional mediation.
• Ghost passkeys. A failed signup or enrollment finish leaves the new passkey in the password manager, and removed passkeys stay too; the script never calls signalUnknownCredential or signalAllAcceptedCredentials (assets/passkey.js:120-157, 297-320).
• One message for every error. Every DOMException reads "cancelled or unavailable", including InvalidStateError, which means this device already has a passkey for the account (passkey.js:52-57). Two overlapping sign-ins with a counter-based key show the "copied passkey" message, which tells the owner to use another passkey although retrying works (passkey/error.rs:179).
• Display names accept direction overrides and zero-width characters: only char::is_control is rejected (passkey/types.rs:57-63).
• The invitation token travels in the query string (/register?invitation=, admin/service.rs:590), unlike every other token, which uses the fragment.
• Deleting requires the handle typed exactly; "@alice" or "Alice" is refused (deletion/mod.rs:271).
• Tabs break each other. Each GitHub or Google start makes a new browser binding for the same cookie, so starting both in two tabs breaks the first (federation/mod.rs:299).

Proposal

Fix each: conditional mediation on /login; the Signal API after failures and on settings; specific messages; reject Unicode format characters (Cf); the invitation in the fragment; normalize the typed handle; reuse an existing browser binding.

Done when

• Each item has a test or a browser check.