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/Flow/NodePanel.tsx

@@ -854,7 +854,7 @@ function ParamsForm({
  * How much of a machine this node takes, and how long it is expected to take.
  *
  * A named size is the usual answer: what "gpu-small" means is a property of the
- * machines this installation has, and those change. The numbers are still there
+ * machines this instance has, and those change. The numbers are still there
  * for a node that genuinely wants its own.
  */
 function ResourcesSection({