feat(desktop): add Velopack auto-update support

Lets the desktop app self-update like CncSender, but only restarts after
the user confirms via a clickable status-bar prompt -- an open, unsaved
nest must never be discarded automatically. The feed URL is a blank-by-
default, per-machine setting (File > Options), since it's operator
infrastructure that doesn't belong in source control; Deploy-OpenNest.ps1
packages releases for whatever feed host/path the operator supplies.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/auto-update.md

@@ -0,0 +1,37 @@
+# Desktop auto-update
+
+The Windows desktop app (`OpenNest.exe`) can self-update via
+[Velopack](https://velopack.io): `OpenNest.UpdaterService` polls a static
+file feed every 30 minutes, downloads a newer release in the background, and
+only restarts to apply it after the user confirms (a clickable status-bar
+item appears once a download finishes). It never force-restarts; an open,
+unsaved nest is never discarded automatically.
+
+## Configuring the feed
+
+The feed URL is a per-machine setting
+(`OpenNest.Properties.Settings.UpdateFeedUrl`), set from **File > Options >
+Auto-update feed URL**. It is blank by default, which disables update checks
+entirely — this is intentional: the feed URL is operator infrastructure, not
+something that belongs in source control, so no default ships in the repo.
+Enter a feed's base URL (`http://host:port`) and restart the app to pick it
+up.
+
+Velopack only applies updates to an installed copy (`UpdateManager.IsInstalled`);
+running from a build output directory never attempts an update.
+
+## Hosting a feed
+
+Any static file server that can serve a `vpk pack` output directory works.
+`scripts/Deploy-OpenNest.ps1` builds a self-contained `win-x64` publish,
+packs it with the `vpk` CLI (`dotnet tool install -g vpk`), and `scp`s the
+result to a directory you provide:
+
+```powershell
+./scripts/Deploy-OpenNest.ps1 -FeedHost <host> -FeedUser <user> -RemoteDataDir <path>
+```
+
+Bump `<Version>` in `OpenNest/OpenNest.csproj` before each run. This is
+independent of [the GitHub release process](releasing.md) and of
+`Publish-Windows.ps1` — the latter always overrides `<Version>` via
+`-p:Version`, so the two don't conflict.