Rename Installation to Instance

Follows the portal: the noun is "instance" everywhere the app says it —
UI strings, CLI output, error details, docs and comments. The wire keys
(`instance_id`, `instance_token`) and the hub route this calls move with it.

An existing cloud.json is adopted rather than refused: without the key
alias the dataclass fails to parse, which the caller swallows and reads as
"never enrolled" instead of "reconnect".

`instance_key` on a node type becomes `target_key`. It means the outside
thing a node points at, which is a different sense of the word, and keeping
both would put two meanings of "instance" in one codebase.

frontend/src/main.tsx

@@ -51,7 +51,7 @@ const isPointlessToRetry = (error: unknown) =>
 const handleApiError = (error: Error) => {
   const offline = offlineDetail(error)
   if (offline) {
-    // The installation is unreachable, not the session invalid: keep the user
+    // The instance is unreachable, not the session invalid: keep the user
     // where they are and let the banner explain.
     connectionStore.setOffline(offline.lastSeen)
     return
@@ -67,7 +67,7 @@ const handleApiError = (error: Error) => {
     if (portal) {
       // The portal knows whether they are still signed in; it can mint a fresh
       // handoff or send them to the login screen.
-      window.location.href = `${portal.portalUrl}?reauth=${portal.installationId}`
+      window.location.href = `${portal.portalUrl}?reauth=${portal.instanceId}`
       return
     }
     safeStorage.remove("access_token")