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>

Cargo.toml

@@ -31,6 +31,7 @@ serde = { version = "1", features = ["derive", "rc"] }
 serde_json = { version = "1", features = ["float_roundtrip"] }
 log = "0.4"
 sha2 = "0.10"
+semver = "1"
 
 # geometry kernel (Apache-2.0; see plan/adr/0001-geometry-kernel.md). Only crates/kernel uses it.
 truck-modeling = "0.6"