fix(desktop): mirror signed releases to public repo for the updater

The updater endpoint pointed at mca/parking_solution's own Gitea "latest
release" redirect, but that repo is private and field appliances have no
Gitea credentials — every update check was silently failing. release.yml
now mirrors signed installers to mca/public_releases (public, installers
only) under a fixed desktop-latest tag; tauri.conf.json points there.
Rejected embedding a read token in the app instead, given the booth-operator
threat model.

Also: make the appliance-provisioning root_directory gotcha impossible to
skim past (boxed callout + explicit next-step pointers), after it caused a
second missed step on the park-2 install.

apps/desktop/README.md

@@ -35,8 +35,16 @@ pnpm --filter @parking/desktop bundle   # build the SPA + bundle the desktop app
 Requires the Rust toolchain and (on Linux) WebKitGTK 4.1 + libsoup-3 dev libraries. Under WSL2 the
 window needs a display (WSLg or an X server).
 
+## Auto-update
+
+Signed updates are built and published by `.gitea/workflows/release.yml` on a `vX.Y.Z` tag, mirrored
+to the public `mca/public_releases` repo (this repo is private; the updater runs on offline-first
+field appliances with no Gitea credentials, so its endpoint must be reachable unauthenticated —
+see that workflow's header and `wiki/decisions/desktop-shell-tauri.md`). The updater config and
+signing pubkey live in `tauri.conf.json`; the private signing key is held outside the repo, never
+committed.
+
 ## Not here (deliberately)
 
-Kiosk lockdown (fullscreen/no-decorations), auto-update, code signing, and launching Fastify from
-the shell are out of scope for the scaffold — on the appliance Fastify runs as its own service and
-this shell connects to it.
+Kiosk lockdown (fullscreen/no-decorations) and launching Fastify from the shell are out of scope for
+the scaffold — on the appliance Fastify runs as its own service and this shell connects to it.