fix(electron): improve protocol URL handling and register URL scheme on Linux

Suppress noisy "EXITING due to failed single instance lock" message when
a second instance is launched via xdg-open for protocol URL handling.
Register superproductivity:// as a MIME type handler in the .desktop file
so Linux users no longer need manual xdg-settings configuration.
Remove duplicate requestSingleInstanceLock() call in start-app.ts.

Closes #173

electron-builder.yaml

@@ -47,6 +47,8 @@ linux:
   # Fix for issue #5259: Set executable name without spaces for snap compatibility
   executableName: superproductivity
   category: Office;ProjectManagement
+  mimeTypes:
+    - x-scheme-handler/superproductivity
   desktop:
     entry:
       StartupWMClass: superproductivity

electron/main.ts

@@ -1,4 +1,5 @@
 import { app } from 'electron';
+import { PROTOCOL_PREFIX } from './protocol-handler';
 import { startApp } from './start-app';
 
 const IS_MAC = process.platform === 'darwin';
@@ -8,7 +9,10 @@ if (!IS_MAC) {
   // because of https://github.com/electron/electron/issues/14094
   const isLockObtained = app.requestSingleInstanceLock();
   if (!isLockObtained) {
-    console.log('EXITING due to failed single instance lock');
+    const hasProtocolUrl = process.argv.some((arg) => arg.startsWith(PROTOCOL_PREFIX));
+    if (!hasProtocolUrl) {
+      console.log('Another instance is already running. Exiting.');
+    }
     // Force immediate exit without waiting for graceful shutdown
     process.exit(0);
   } else {

electron/start-app.ts

@@ -51,19 +51,9 @@ let mainWin: BrowserWindow;
 let idleTimeHandler: IdleTimeHandler;
 
 export const startApp = (): void => {
-  // Initialize protocol handling
+  // Initialize protocol handling (registers second-instance listener for URL forwarding)
   initializeProtocolHandling(IS_DEV, app, () => mainWin);
 
-  // Handle single instance lock (not needed on macOS - OS handles it natively,
-  // and the singleton socket is blocked by the App Store sandbox)
-  if (!IS_MAC) {
-    const gotTheLock = app.requestSingleInstanceLock();
-    if (!gotTheLock) {
-      app.quit();
-      return;
-    }
-  }
-
   // LOAD IPC STUFF
   initIpcInterfaces();