Sync and authenticate with SSH keys on the client

Completes the client half of SSH keys: they sync alongside hosts, appear in
their own list, and can be selected to authenticate a connection instead of
typing a password.

The reconciler and the repository were Host-typed throughout, so the choice was
to generalise them or to keep a second copy per item type. Generalised, because
ItemReconciler's whole premise is that the pull and the push paths must answer
the same collision the same way — two copies would drift the first time one of
them was fixed. What is genuinely per-type now arrives through
IItemKind<TSecret>: the cipher, the merge, the plaintext columns, and the noun
to use when telling a person what happened to their item. Generic where the
server's IItemKind is not, and for the reason that reverses there — the client
needs the concrete type, because it merges field by field.

The pull filter is derived from the same registry that builds the reconcilers.
That is the specific failure being designed out: an item type that encrypts,
merges and lists perfectly and is never once requested from the server, so it
works on the machine that made it and exists nowhere else.

No client cache migration. The item table's primary key and the outbox's unique
index already carry the entity type, and AadResourceTypes already mapped SshKey
— so a host and a key may share an id and never see each other's rows, which
SshKeySyncTests now arranges deliberately.

A key hands the server nothing in plaintext. There is a public_key_fingerprint
column and it would be accepted; leaving it null is deliberate. A fingerprint is
not secret but it is a stable identifier for a key pair, so filling it would let
an operator tell which of their users hold the same key and correlate one across
vaults, for a column nothing reads. The design allows itself one plaintext
concession — the relay address, which the relay cannot work without — and this
is not that.

A key is chosen per connection rather than bound to a host, which works the way
ssh -i does. Binding one needs a field on HostSecret and therefore a payload
schema bump, which makes every host written afterwards read-only on an older
build; worth doing deliberately rather than as a side effect of adding keys.

Three things this found, all of them by being falsified rather than by review:

- Making the reconciler generic silently turned a record comparison into
  reference equality, because == on a type parameter is not value equality. The
  effect would have been a conflict recorded on every pass for an unacknowledged
  create that had in fact landed. Sabotaging the fix left all 73 tests passing —
  nothing covered that branch — so ConflictMatrixTests now has
  AnUnacknowledgedCreateThatDidLand_IsDroppedQuietly, which fails without it.

- A test asserting that a blank passphrase reaches SSH.NET as null was vacuous:
  it exercised the editor, not the credential path, and passed with the guard
  deleted. Resolved by making SshKeySecret.Passphrase normalise an empty string
  to null, so there is one spelling of one state — which also keeps two clients
  from producing different payload bytes for an identical key. That exposed a
  wider gap: SshKeySecret, its codec and its merge had no direct unit tests at
  all. They have 25 now.

- The reason first given for that normalisation was false. It claimed SSH.NET
  rejects a passphrase supplied for an unprotected key; measured against a real
  sshd it ignores it and authenticates anyway. Corrected everywhere it was
  stated and recorded in docs/platform-flags.md. The same test file also closes
  a real hole: SshPrivateKeyCredential had never been exercised against a
  server, because the existing key test builds SSH.NET's auth method directly
  and bypasses the path a vault-held key actually takes.

Only one editor may be open at a time. Both sit in the same 340-pixel column as
Auto rows and their heights together exceed it at the window's minimum size, so
two open editors put the lower one's Save and Cancel past the bottom edge — the
same failure this window already shipped once with the setup screens. Expressed
as a state rule because that is the only form of it this repository can check:
nothing here loads a .axaml. The refusal keeps what was typed, since in the key
editor that is a pasted private key the user may have nowhere else.

The end-to-end slice now carries a key as well as a host, so both item types go
through the real API, the real PostgreSQL and the real crypto in one pass — the
three hand-kept mappings between enums that do not line up are the reason that
is worth doing rather than trusting the unit suites.

735 tests green, including the container-backed SSH and end-to-end suites. Zero
warnings, dotnet format clean.

README.md

@@ -127,10 +127,15 @@ In the app, enter `http://localhost:5233` as the server. Your browser opens for
 skipped and cannot be recovered from the server. You can then add a host and open a shell on it. Keycloak's
 admin console is at `http://localhost:18080` (`admin` / `admin`).
 
-Three of M1's known gaps are visible immediately, so they are worth expecting rather than diagnosing: a
-connection asks for the host's password every time, because credentials are not a synced entity type yet;
-host key trust lasts one session, because known hosts do not live in the vault yet; and unlock asks for the
-passphrase on every launch, because no device key is registered.
+You can also add an SSH key, which is stored in the vault like a host and synced the same way: paste the
+private key, tick **Use key** next to Connect, and the selected key authenticates instead of a password.
+
+Three of M1's known gaps are visible immediately, so they are worth expecting rather than diagnosing:
+password authentication asks for the password every time, because credentials are not a synced entity type
+yet (keys are — passwords are not); a key is chosen per connection rather than remembered per host, because
+binding one to a host needs a new field on the host payload and so a schema version bump; and unlock asks
+for the passphrase on every launch, because no device key is registered. Host key trust also lasts one
+session, because known hosts do not live in the vault yet.
 
 ### End-to-end verification
 
@@ -197,9 +202,11 @@ off-Windows.
   [End-to-end verification](#end-to-end-verification).
 
   Known gaps in the client, stated rather than implied by the interface: credentials are not a synced
-  entity type yet, so a connection still asks for a password; known host keys live in memory for one
-  session instead of in the vault; and no device key is registered, so the passphrase is needed on every
-  launch until the OS keystore is wired.
+  entity type yet, so password authentication still asks for the password each time — SSH keys *are*
+  synced, and are the way to connect without typing anything; a key is picked per connection rather than
+  bound to a host, which needs a field on the host payload and therefore a schema version bump; known host
+  keys live in memory for one session instead of in the vault; and no device key is registered, so the
+  passphrase is needed on every launch until the OS keystore is wired.
 - **M2 — full personal vault**, robust sync, relay.
 - **M3 — teams**, sharing, ACLs.
 - **M4 — hardening and ops**, packaging, self-hosting guide.