[desktop] (Maybe) fix the dock icon on Linux (#1959)

Ref: https://github.com/ente-io/ente/issues/1909

build/icons/512-512.png is just a 1/2 copy of build/icons.png.

desktop/src/main.ts

@@ -154,6 +154,8 @@ const createMainWindow = () => {
         backgroundColor: "black",
         // We'll show it conditionally depending on `wasAutoLaunched` later.
         show: false,
+        // (Maybe) fix the dock icon on Linux.
+        ...windowIconOptions(),
     });
 
     const wasAutoLaunched = autoLauncher.wasAutoLaunched();
@@ -313,6 +315,53 @@ const setupTrayItem = (mainWindow: BrowserWindow) => {
     tray.setContextMenu(createTrayContextMenu(mainWindow));
 };
 
+/**
+ * On Linux the app does not show a dock icon by default, attempt to fix this by
+ * returning the path to an icon as the "icon" property that can be passed to
+ * the BrowserWindow during creation.
+ */
+const windowIconOptions = () => {
+    if (process.platform != "linux") return {};
+
+    // There are two, possibly three, different issues with icons on Linux.
+    //
+    // Firstly, the AppImage itself doesn't show an icon. There does not seem to
+    // be a reasonable workaround either currently. See:
+    // https://github.com/AppImage/AppImageKit/issues/346
+    //
+    // Secondly, and this is the problem we're trying to fix here, when the app
+    // is started it does not show a dock icon (Ubuntu 22) or shows the generic
+    // gear icon (Ubuntu 24). The issue possibly exists on other distributions
+    // too.
+    //
+    // Electron provides a `BrowserWindow.setIcon` function which should solve
+    // our issue, we could call it selectively on Linux. There is also an
+    // apparently undocumented "icon" option that can be passed when creating a
+    // new BrowserWindow, and that is what most of the other code I saw on
+    // GitHub seems to be doing.
+    //
+    // However, try what I may, I can't get either of these to work. Which leads
+    // me to believe there is a third issue: I can't get it to work because I'm
+    // testing on an Ubuntu 24 VM, where this might just not be working:
+    // https://askubuntu.com/questions/1511534/ubuntu-24-04-skype-logo-on-the-dock-not-showing-skype-logo
+    //
+    // 24 isn't likely the year of the Linux desktop either.
+    //
+    // For now, I'm adding a very specific incantation taken from
+    // https://github.com/arduino/arduino-ide/blob/main/arduino-ide-extension/src/electron-main/fix-app-image-icon.ts
+    //
+    // Possibly all this specific naming of the file etc is superstition, and
+    // just any name would do as long as the path is correct, but let me try it
+    // this way and see if this gets the icon to appear on Ubuntu 22 etc.
+
+    const icon = path.join(
+        isDev ? "build" : process.resourcesPath,
+        "icons/512x512.png",
+    );
+
+    return { icon };
+};
+
 /**
  * Older versions of our app used to maintain a cache dir using the main
  * process. This has been removed in favor of cache on the web layer. Delete the