sync: what the on-device round found

Everything here came from running the account flow against a real
Nextcloud rather than from reading the code.

Discovery
- A typed bare origin now gets the RFC 6764 well-known probe. It was
  returned as the only candidate, so `https://cloud.example.com` — what
  people actually type — was PROPFIND'd against the web UI, answered 405,
  and a working Nextcloud reported as "not a CalDAV server".
- A same-host HTTPS→HTTP redirect is put back on TLS instead of refused
  (`dav` change 7). A Nextcloud behind a TLS-terminating proxy without
  `overwriteprotocol` builds every redirect with http://, including the
  /.well-known/caldav hop discovery depends on. Cross-host still throws.
- Outcomes carry a `Cause` the UI translates, not the server's own words.
  "HTTP 405 Method Not Allowed" told someone entering an address nothing,
  in a language they may not read, from outside strings.xml.
- An IPv6 origin keeps its brackets: `HttpUrl.host` returns "fd00::1", so
  the rebuilt origin did not parse and a homelab address came back as
  "not an address".

Login Flow v2
- The poll response's scheme is coerced, never refused. Nextcloud returns
  the app password exactly once, so throwing there burned a live
  credential and left it dangling in the user's device list. The host
  mismatch already worked this way; the scheme now matches it.

Accounts
- The accounts screen observes Room and the sign-in state instead of
  taking a snapshot, so a sync landing — or a 401 stopping an account —
  reaches a screen that is already open.
- A per-account detail screen, and provider identity (`CalDavProvider`)
  shared with the quirk table so one list drives both the icon and the
  warning.
- The password field masks: floret-kit's `InlineTextField` gained a
  visual transformation, since `KeyboardType.Password` only tells the IME
  to drop suggestions.

.gitignore

@@ -69,3 +69,7 @@ Thumbs.db
 # Local agent notes: machine-specific build setup and on-device rules, not
 # anything the project itself depends on.
 /CLAUDE.md
+
+# Scratch backlog. Says so in its own header — dumped items get turned into
+# real work, not committed as a list.
+/req_changes.md