Let a host marked for the relay actually be reached through it

The relay checkbox said it was not wired up, and the plan said the server
half was built and shipped. It was not: there was ADR 0004, the contract
records, RelayOptions and a CHECK constraint, and RelayOptions said in its
own remarks that there was no endpoint and no dialler. So both halves are
built here, as step 3 of docs/reaching-a-host-you-cannot-dial.md.

Server, in Features/Relay:

- POST /api/v1/relay/tickets makes every decision: the host, Connect on
  its vault, the relay switched on, and the stored name resolved once and
  judged by RelayAddressPolicy. Loopback, link-local, 0.0.0.0/8,
  multicast and the metadata endpoints are refused with no configuration
  escape, with IPv4-mapped and NAT64 spellings unwrapped first; then the
  operator's denials, allow-list and private-network switch. What passes
  goes into a Data Protection ticket, and nothing resolves the name again.
  Every target refusal is the same 403, so the endpoint cannot be used to
  map the operator's DNS; the real reason is logged.
- GET /api/v1/relay/connect takes the ticket as a second subprotocol,
  checks its expiry against TimeProvider and spends it with a unique
  insert into relay_ticket_use before accepting the upgrade. After the
  upgrade a refusal is a close code: 4502 for a host the server could not
  reach, 4429 for a limit, 4408 idle, 4409 lifetime, 1001 after a drain.
- relay_session is the audit row, written from short-lived scopes.

Client:

- RelayBridge is a loopback SOCKS5 listener that accepts once and pipes
  to the relay socket. It rides on the new SshLoopbackProxy.Lifetime, so
  the connection disposes it on every way a session ends and a failed
  connect disposes it before throwing.
- ServerRelay, exposed as IVaultServer.Relay from /api/v1/meta, asks for
  the ticket, opens the socket, starts the bridge and turns every refusal
  into a sentence.
- VaultViewModel and TransfersViewModel reach a relayed host through it,
  never fall back to dialling it directly, and replace SSH.NET's transport
  error with the relay's own reason when it gave one.

Neither side ever cancels a WebSocket operation. Cancelling one aborts
the socket, and an aborted socket sends no close frame, which is the only
way the client learns the server could not reach the host.
AnIdleSession_IsClosedWithTheIdleCode asserts that the code arrives.

Three departures from ADR 0004, recorded in it: awaited copy loops rather
than Pipelines (same backpressure, two 32 KiB buffers), no ?ticket=
fallback, and no device id in the ticket.

Tests: API 245, shell 582 (5 skipped), layout 170, SSH 110 including a
real login through the bridge, API client 30, contracts 25. Not run: the
Android build (its change is copy only) and the Alpine layout oracle.

docs/design-import-gaps.md

@@ -254,7 +254,7 @@ the chrome, hosts and terminals, file transfer, the vault, teams, and preference
 > | v5 element | What ships instead |
 > | --- | --- |
 > | The status dot's third, amber state | ◆ **Still two states, on the same reasoning v3 and v4 both gave and v5 was asked to reconsider.** Green means a terminal is open on that host, grey means one is not; nothing here pings a machine, so the amber "reachable but not connected" state has no fact behind it to draw. |
-> | The relay checkbox's mock copy | The pre-existing sentence, unchanged, "(not wired up yet)" included — restyled into the mock's own nested-card shape (radius 12, a title beside the box rather than under it) but not reworded. The mock's own copy implies a relay this client can dial; it cannot, for the reason `HostDrawer.axaml`'s own remarks give. |
+> | The relay checkbox's mock copy | Restyled into the mock's own nested-card shape (radius 12, a title beside the box rather than under it). The "(not wired up yet)" it carried while this client could not dial the relay came out when it could; the sentence under it now says what ticking it does — the server dials the host, so the server has to be able to reach it, and the address is stored in plain text. |
 > | The host card as a link straight to a terminal | Click still selects, double-click still connects, and the pencil still opens the pane — the mock's card-as-link is not adopted, because multi-select (Ctrl, Shift, the marquee band) depends on a plain click meaning "choose this one" rather than "go". |
 > | Quick connect's SSH/SFTP kind column | The auth word — credential, key, or password — see above. |
 > | A collapsed section staying collapsed after a restart | In memory only, for the running session. `settings.json` holds two scalars by decision — the terminal's text size and whether this machine checks for updates on its own — and collapse state is not judged worth a third. |