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>

crates/ui-egui/src/toolbar.rs

@@ -250,7 +250,9 @@ pub fn app_bar(app: &mut SolveApp, ui: &mut egui::Ui) {
         let captions = if app.custom_titlebar { crate::titlebar::WIDTH } else { 0.0 };
         let tabs_start = x + 16.0;
         let (report, report_label) = report_rect(r, captions, tabs_start);
-        let tabs_end = report.left() - 8.0;
+        // A newer release: its Update button, left of Report a bug (`update`).
+        let update = crate::update::button(app, ui, report.left() - 6.0, report.top(), report_label);
+        let tabs_end = update.map_or(report.left(), |u| u.left()) - 8.0;
         // Narrow title bars can fit only part of the tab strip. Its hit regions must not
         // extend into the persistent report and window controls.
         let tabs_rect = Rect::from_min_max(pos2(tabs_start.min(tabs_end), r.top()), pos2(tabs_end, r.bottom()));
@@ -296,7 +298,7 @@ pub fn app_bar(app: &mut SolveApp, ui: &mut egui::Ui) {
         ui.painter().text(
             pos2(r.right() - 12.0 - captions, r.center().y),
             Align2::RIGHT_CENTER,
-            format!("v{}", env!("CARGO_PKG_VERSION")),
+            format!("v{}", crate::update::current_version()),
             FontId::proportional(11.0),
             Color32::from_rgb(160, 168, 182),
         );