App 2.18.0: install from a USB stick

Settings -> App -> "Install from USB stick" (car build only). It looks
in the top folder of every mounted volume (found from the app's own
folder on it, since the head unit has no file picker) for
NilsMG4-unstable-<version>.apk, takes the newest one newer than the
installed version, checks it against its .sha256 sidecar and the
package name, and installs it with the same PackageInstaller session
as the over-the-air update. The stable build gets a stub and no button.

Tested on the emulator: 2.18.0 installed a 2.18.1 from the storage's
top folder without a confirmation, as the system user, so the OTA
install step should not need one either.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QLJfZQZeAKAwSqViTmLzki

SPECS.md

@@ -112,6 +112,11 @@ tyre pressures, GPS (lat, lon, elevation, heading) and the is_* flags. It does *
   INGEST_TOKEN) into git and pushed it. Commit amended and force-pushed; both values regenerated in
   the now gitignored `k8s/secrets.yaml` (since moved to `k8s/secrets/secrets.yaml`, outside
   `kubectl apply -f k8s/`). Filled-in secrets live only there from now on.
+- 2026-10-18 — App 2.18.0: Settings → App → "Install from USB stick" (car build only): the newest
+  `NilsMG4-unstable-<version>.apk` newer than the installed one in a stick's top folder, checked
+  against its `.sha256` and the package, installed with the OTA installer session. Tested on the
+  emulator: 2.18.0 installed a 2.18.1 from the storage's top folder silently, as the system user —
+  so the OTA install step should not need a confirmation either.
 - 2026-10-18 — App 2.17.5: Server tab fits the car's window. The "Your private connection" card is
   gone (its text was out of date); Import from file sits with Save and Test in one row; the
   installation ID is shown as text, not edited, since it is derived from the head unit (a config