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/components/Sidebar/AppSidebar.tsx

@@ -33,7 +33,7 @@ import { type Item, Main } from "./Main"
 /**
  * Where the documentation is published.
  *
- * A constant rather than something derived: an installation reached over a LAN
+ * A constant rather than something derived: an instance reached over a LAN
  * address, or at localhost, has no domain to build this from, and the docs are
  * one site for all of them.
  */
@@ -70,13 +70,13 @@ export function AppSidebar() {
     ? [...baseItems, { icon: Users, title: "Admin", path: "/admin" }]
     : baseItems
 
-  // Reached through a portal, the way out is back to the installations list —
+  // Reached through a portal, the way out is back to the instances list —
   // and the session belongs to the portal, so logging out is its business.
   const items: Item[] = portal
     ? [
         {
           icon: ArrowLeft,
-          title: "All installations",
+          title: "All instances",
           onClick: () => window.location.assign(portal.portalUrl),
         },
         ...withAdmin,