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/lib/connectionStore.ts

@@ -1,5 +1,5 @@
 /**
- * Whether the installation this page talks to is reachable right now.
+ * Whether the instance this page talks to is reachable right now.
  *
  * Transport state, kept apart from `liveStore` on purpose: that one holds what
  * the engine is doing, this one holds whether we can hear it at all. Only
@@ -11,7 +11,7 @@
 
 type Connection = {
   offline: boolean
-  /** When the portal last heard from the installation, epoch ms. */
+  /** When the portal last heard from the instance, epoch ms. */
   lastSeen: number | null
 }
 
@@ -38,7 +38,7 @@ function emit(next: Connection) {
 
 export const connectionStore = {
   setOffline(lastSeen: string | number | null) {
-    // The socket is up, so the installation is reachable: this was one bad
+    // The socket is up, so the instance is reachable: this was one bad
     // answer rather than a lost tunnel, and the banner would have nothing to
     // take it back down again.
     if (socketOpen) return
@@ -56,7 +56,7 @@ export const connectionStore = {
   /**
    * The live socket opened or closed.
    *
-   * An open socket is proof the installation can be heard, so it also clears
+   * An open socket is proof the instance can be heard, so it also clears
    * the banner — and since the socket reconnects on its own, that is the one
    * release the offline state can always count on.
    */
@@ -76,7 +76,7 @@ export const connectionStore = {
 /**
  * The proxy's offline answer: a 503 carrying `{offline, last_seen}`.
  *
- * Told apart from every other 503 by that body, so an installation that is
+ * Told apart from every other 503 by that body, so an instance that is
  * merely busy is not reported as unreachable.
  */
 export function offlineDetail(