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/vite.config.ts

@@ -7,7 +7,7 @@ import { defineConfig } from "vite"
 // https://vitejs.dev/config/
 export default defineConfig({
   // A portal serves this bundle under a path of its own and caches one copy
-  // for every installation, so asset URLs have to be absolute under that
+  // for every instance, so asset URLs have to be absolute under that
   // prefix. Unset — every ordinary build — this stays "/".
   base: process.env.VITE_BASE || "/",
   resolve: {