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/routes/_layout/alerts.tsx

@@ -97,7 +97,7 @@ function showSetting(value: unknown): string {
 /**
  * The one channel whose setting is on the device rather than in the config:
  * a browser has to ask its own permission, and what it hands back is stored
- * against this installation.
+ * against this instance.
  */
 function ThisBrowser() {
   const { showSuccessToast, showErrorToast } = useCustomToast()

frontend/src/routes/_layout/index.tsx

@@ -33,7 +33,7 @@ const HEADER =
  * it has been doing. The brain and the health screens compose in here rather
  * than living at routes of their own.
  *
- * Top to bottom it is a widening lens: the whole installation as a graph, what
+ * Top to bottom it is a widening lens: the whole instance as a graph, what
  * has been built on it, whether it is well, then flow by flow and finally
  * moment by moment.
  */

frontend/src/routes/_layout/settings.tsx

@@ -50,7 +50,7 @@ function UserSettings() {
         <motion.div variants={slideUp}>
           <Appearance />
         </motion.div>
-        {/* Whether this whole installation can be reached from outside is not
+        {/* Whether this whole instance can be reached from outside is not
             a personal preference, so it only exists for an operator. */}
         {currentUser.is_superuser && (
           <motion.div variants={slideUp}>