Tell people when a newer release is out (#1100)

At start, and at most once a day, a background thread asks GitHub for the latest stable
release (one HTTPS GET to releases/latest, nothing else sent; 15 s timeout; silent when
offline; GitHub's rate limit respected). When it is newer than this build, an Update button
appears next to Report a bug. It opens a window with the version, the release notes and
Download (the asset that fits this installation), Skip this version and Remind me later.

- Versions compare as semver; a nightly counts as the release it was built after, so it is
  offered only a newer stable release.
- The installation is detected (AppImage, Flatpak, Snap from the store or a file, deb, rpm,
  tar.gz, Windows MSI/portable/exe, macOS app); Download picks the fixed-name asset.
  Development builds and the Flatpak (no network permission) never check.
- Preferences > General > Updates: Check for updates automatically (on) and Check now.
- HTTP via ureq + rustls with the operating system's certificates (desktop app only).
- docs: Updates and privacy page.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/releasing.md

@@ -20,7 +20,8 @@ also attached without the version in their names, for example `solidcraft-window
 `solidcraft-linux-x86_64.AppImage`, and listed in `SHA256SUMS.txt`. The README's Download section
 links to `https://github.com/bherbruck/solidcraft/releases/latest/download/<name>`, which follows
 the release marked latest; nightlies never are. Keep these names stable: renaming one breaks the
-README links.
+README links and the in-app update check, which picks them by name (`crates/ui-egui/src/update.rs`;
+[updates and privacy](book/src/guide/updates.md)).
 
 Project Pages publishes the documentation, browser app and published-release Flatpak
 repository under `/solidcraft/`. Do not add a separate owner-site publisher or permanent