feat(linux): native build path + Linux-specific launcher optimizations

- X-Plane auto-detection now finds Steam (native + Flatpak), /opt and
  external-drive SteamLibrary installs, not just ~/ and macOS/Windows paths.
- Close-to-background minimizes on Linux instead of hiding to tray, so the app
  is never stranded on desktops without a tray (e.g. vanilla GNOME).
- Disable WebKitGTK's DMABUF renderer on Linux to avoid black/blank windows on
  some GPU/driver combos (overridable via the env var).
- Launcher font stack gains Linux UI/mono fallbacks (Cantarell/Ubuntu/Noto).
- scripts/build.sh: Docker-free Linux AppImage build — repack the cockpit into
  the prebuilt bundle for web-only changes; recompile natively for code changes.

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

desktop/src-tauri/src/lib.rs

@@ -66,30 +66,71 @@ fn suggest_port(start: u16) -> u16 {
     start
 }
 
+// A directory is an X-Plane 12 root iff it holds Resources/default data.
+fn is_xplane_root(p: &std::path::Path) -> bool {
+    p.join("Resources").join("default data").is_dir()
+}
+
 #[tauri::command]
 fn default_xplane_path() -> Option<String> {
     let home = std::env::var("HOME")
         .or_else(|_| std::env::var("USERPROFILE"))
         .unwrap_or_default();
-    let candidates = [
+    let user = std::env::var("USER")
+        .or_else(|_| std::env::var("USERNAME"))
+        .unwrap_or_default();
+
+    let mut candidates = vec![
+        // Generic / cross-platform
         format!("{home}/X-Plane 12"),
         format!("{home}/Desktop/X-Plane 12"),
+        format!("{home}/Games/X-Plane 12"),
+        // Steam on Linux — native, plus the Flatpak Steam sandbox path
+        format!("{home}/.steam/steam/steamapps/common/X-Plane 12"),
+        format!("{home}/.local/share/Steam/steamapps/common/X-Plane 12"),
+        format!("{home}/.var/app/com.valvesoftware.Steam/.local/share/Steam/steamapps/common/X-Plane 12"),
+        // System-wide (Linux)
+        "/opt/X-Plane 12".to_string(),
+        // macOS
         "/Applications/X-Plane 12".to_string(),
+        // Windows
         "C:/X-Plane 12".to_string(),
         "D:/X-Plane 12".to_string(),
     ];
+
+    // Secondary/external drives: Steam libraries on extra disks usually mount
+    // under /media/$USER, /run/media/$USER or /mnt. Probe each mounted volume
+    // for the standard SteamLibrary layout (and a bare "X-Plane 12" folder).
+    for base in [
+        format!("/media/{user}"),
+        format!("/run/media/{user}"),
+        "/mnt".to_string(),
+    ] {
+        if let Ok(entries) = std::fs::read_dir(&base) {
+            for vol in entries.flatten().map(|e| e.path()) {
+                candidates.push(
+                    vol.join("SteamLibrary/steamapps/common/X-Plane 12")
+                        .to_string_lossy()
+                        .into_owned(),
+                );
+                candidates.push(
+                    vol.join("steamapps/common/X-Plane 12")
+                        .to_string_lossy()
+                        .into_owned(),
+                );
+                candidates.push(vol.join("X-Plane 12").to_string_lossy().into_owned());
+            }
+        }
+    }
+
     candidates
         .into_iter()
-        .find(|c| PathBuf::from(c).join("Resources").join("default data").is_dir())
+        .find(|c| is_xplane_root(&PathBuf::from(c)))
 }
 
 #[tauri::command]
 fn valid_xplane_path(path: String) -> bool {
-    !path.is_empty()
-        && PathBuf::from(&path)
-            .join("Resources")
-            .join("default data")
-            .is_dir()
+    !path.is_empty() && is_xplane_root(&PathBuf::from(&path))
 }
 
 #[tauri::command]
@@ -225,11 +266,17 @@ pub fn run() {
             build_tray(app.handle())?;
             Ok(())
         })
-        // Closing the window hides it instead of quitting, so the server keeps
-        // serving tablets in the background. Quit from the tray.
+        // Closing the window keeps the app alive so the server keeps serving
+        // tablets in the background. On macOS/Windows the tray icon is always
+        // reachable, so fully hide. On Linux many desktops (notably vanilla
+        // GNOME) show no tray at all, so hiding would strand the app with no way
+        // back — minimize instead, keeping it in the taskbar while it runs.
         .on_window_event(|window, event| {
             if let tauri::WindowEvent::CloseRequested { api, .. } = event {
                 api.prevent_close();
+                #[cfg(target_os = "linux")]
+                let _ = window.minimize();
+                #[cfg(not(target_os = "linux"))]
                 let _ = window.hide();
             }
         })

desktop/src-tauri/src/main.rs

@@ -2,5 +2,14 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
 
 fn main() {
+    // WebKitGTK's DMABUF renderer shows a black/blank window on a number of Linux
+    // GPU/driver combinations (notably Nvidia and older Mesa). This launcher is a
+    // simple control panel, so favour reliability over GPU compositing: disable
+    // the DMABUF renderer unless the user has already set the variable themselves.
+    #[cfg(target_os = "linux")]
+    if std::env::var_os("WEBKIT_DISABLE_DMABUF_RENDERER").is_none() {
+        std::env::set_var("WEBKIT_DISABLE_DMABUF_RENDERER", "1");
+    }
+
     xplane_cockpit_lib::run()
 }