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/Dashboard/PanelsDialog.tsx

@@ -51,7 +51,7 @@ export function PanelsDialog() {
   const { data: config } = useQuery(panelsQueryOptions())
   const { data: dashboards } = useQuery(dashboardsQueryOptions())
   const { user } = useAuth()
-  // Where a screen that cannot reach this installation pairs instead. The
+  // Where a screen that cannot reach this instance pairs instead. The
   // issuer is the portal as a browser reaches it, which is not always the
   // address this machine dialled — enrolment may have named a container.
   const { data: cloud } = useQuery({
@@ -60,12 +60,12 @@ export function PanelsDialog() {
       (await CloudService.readStatus()) as {
         enrolled: boolean
         issuer: string | null
-        installation_id: string | null
+        instance_id: string | null
       },
   })
   const remoteHost =
-    cloud?.enrolled && cloud.issuer && cloud.installation_id
-      ? `${cloud.issuer.replace(/\/$/, "")}/i/${cloud.installation_id}`
+    cloud?.enrolled && cloud.issuer && cloud.instance_id
+      ? `${cloud.issuer.replace(/\/$/, "")}/i/${cloud.instance_id}`
       : ""
   const save = useSavePanels()
   const { showErrorToast } = useCustomToast()
@@ -75,7 +75,7 @@ export function PanelsDialog() {
 
   const panels = config?.panels ?? []
   const known = dashboards?.data ?? []
-  // The installation's own address, not this browser's: administering through
+  // The instance's own address, not this browser's: administering through
   // the portal puts the page on the portal's origin, and a screen cannot be
   // sent there — it has no portal session and could not hold a panel
   // credential if it had one.
@@ -98,7 +98,7 @@ export function PanelsDialog() {
         <DialogTitle>Panels</DialogTitle>
         <DialogDescription>
           A panel is one screen and the dashboards it shows. Point the device at
-          a link and it asks for a code you enter here — this installation's own
+          a link and it asks for a code you enter here — this instance's own
           address for a screen on your network, or the portal's for one hanging
           where this machine is not reachable.
         </DialogDescription>
@@ -187,9 +187,9 @@ function PanelRow({
   onRemove,
 }: {
   panel: PanelDef
-  /** Where this installation answers, as it knows itself. */
+  /** Where this instance answers, as it knows itself. */
   host: string
-  /** Where the portal serves this installation, when it is enrolled. */
+  /** Where the portal serves this instance, when it is enrolled. */
   remoteHost: string
   /**
    * Whether this account may change anything here. Every control below saves
@@ -335,7 +335,7 @@ function PanelRow({
         <Input
           readOnly
           value={link}
-          placeholder="This installation has no address set"
+          placeholder="This instance has no address set"
           aria-label={`Link for ${panel.id}`}
           className="text-muted-foreground"
           onFocus={(event) => event.currentTarget.select()}