chore(frontend): bump APP_VERSION to 18 for SW cache-bust

Service worker evicts old caches on activate, so users always
get the new usb-deployer bundle with the read-pump fix instead
of a stale cached one.

frontend/src/lib/version.ts

@@ -3,4 +3,4 @@
  * Both Footer.svelte and service-worker.ts import from here.
  * Bump this number after every feature/bugfix release.
  */
-export const APP_VERSION = 17;
+export const APP_VERSION = 18;