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.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015YrQnKV3bnQd4K342y8tKj

frontend/public/sw.js

@@ -5,7 +5,7 @@
  * so caching it would hand the next visitor somebody else's session.
  *
  * `registration.scope` is the app's root in both places it runs — `/` on an
- * installation of its own, `/i/{id}/` through the portal — which is why the
+ * instance of its own, `/i/{id}/` through the portal — which is why the
  * push payload does not carry a URL.
  */
 
@@ -16,7 +16,7 @@ self.addEventListener("push", (event) => {
     self.registration.showNotification(alert.title || "Fluksio", {
       body: alert.body || "",
       // Under the portal the icons are the shared bundle's, not this
-      // installation's path.
+      // instance's path.
       icon: scope.pathname.startsWith("/i/")
         ? "/app-shell/icon-192.png"
         : `${scope.pathname}icon-192.png`,
@@ -35,7 +35,7 @@ self.addEventListener("notificationclick", (event) => {
     self.clients
       .matchAll({ type: "window", includeUncontrolled: true })
       .then((clients) => {
-        // A tab on this installation is already open: raise it rather than
+        // A tab on this instance is already open: raise it rather than
         // opening a second one.
         const open = clients.find((client) => client.url.startsWith(scope))
         if (open) return open.focus()