fix(updater): disable in-app updater inside Flatpak sandbox, closes #4440 (#4507)

Flatpak mounts the app directory read-only, so the bundled Tauri updater
can download a new version but never apply it, leaving the user stuck on
the old build with no working install path. Update management belongs to
the Flatpak runtime / system package manager.

Detect the sandbox via FLATPAK_ID or /.flatpak-info and fold it into the
existing `updater_disabled` flag, which propagates to `hasUpdater` and
suppresses the in-app updater window. Release notes still surface as an
informational-only path.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/readest-app/src-tauri/src/lib.rs

@@ -415,8 +415,18 @@ pub fn run() {
             #[cfg(not(target_os = "linux"))]
             let is_appimage = false;
 
+            // Flatpak mounts the app directory read-only, so the bundled updater can
+            // download but never apply an update. Disable it and leave updates to the
+            // Flatpak runtime. Detect via FLATPAK_ID or the /.flatpak-info sandbox file.
             #[cfg(desktop)]
-            let updater_disabled = std::env::var("READEST_DISABLE_UPDATER").is_ok();
+            let updater_disabled = {
+                #[cfg(target_os = "linux")]
+                let is_flatpak = std::env::var("FLATPAK_ID").is_ok()
+                    || std::path::Path::new("/.flatpak-info").exists();
+                #[cfg(not(target_os = "linux"))]
+                let is_flatpak = false;
+                std::env::var("READEST_DISABLE_UPDATER").is_ok() || is_flatpak
+            };
             #[cfg(not(desktop))]
             let updater_disabled = false;