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>

assets/icons/ui/source/icons.rs

@@ -266,6 +266,12 @@ pub fn paint(p: &Painter, r: Rect, name: &str, ink: Color32, fill: Color32, acce
                 pen.line(&[(17.0, y), (21.0, y)], ink);
             }
         }
+        // Update: an arrow down onto a tray (a newer release can be downloaded).
+        "update" => {
+            pen.line(&[(12.0, 3.5), (12.0, 14.5)], ink);
+            pen.line(&[(7.5, 10.0), (12.0, 14.5), (16.5, 10.0)], ink);
+            pen.line(&[(4.0, 15.0), (4.0, 20.0), (20.0, 20.0), (20.0, 15.0)], ink);
+        }
         // Help: a question mark in a ring.
         "help" => {
             pen.circle(12.0, 12.0, 9.0, Color32::TRANSPARENT, ink);