Add single-instance enforcement to Electron app

- Implement app.requestSingleInstanceLock() to prevent multiple instances
- Add user-friendly dialog when second instance is attempted
- Focus and restore existing window instead of creating new instance
- Prevent database corruption and resource conflicts
- Update documentation with single-instance behavior details

BUILDING.md

@@ -246,6 +246,24 @@ npm run build:electron:deb:prod
 npm run build:electron:dmg:prod
 ```
 
+### Single Instance Enforcement
+
+The Electron app enforces single-instance operation to prevent:
+- Database corruption from multiple instances
+- Resource conflicts and performance issues
+- User confusion from multiple windows
+
+**Behavior:**
+- Only one instance can run at a time
+- Attempting to launch a second instance shows a user-friendly dialog
+- The existing window is focused and restored if minimized
+- Second instance exits gracefully
+
+**Implementation:**
+- Uses Electron's `requestSingleInstanceLock()` API
+- Handles `second-instance` events to focus existing window
+- Shows informative dialog explaining why only one instance is allowed
+
 #### Direct Script Usage
 
 ```bash