fix(electron): override draw.io iframe beforeunload to allow window close (fixes #815)

The draw.io iframe registers a window.onbeforeunload handler that returns
a non-empty string whenever its internal editor.modified flag is true.
After the user edits text in a shape, that flag is set and never cleared.

Per Electron BrowserWindow docs, returning a non-void value from any
beforeunload handler in the page tree silently cancels the window close
without showing a dialog. This is what caused the X button (and Cmd+Q)
to do nothing for users who had typed in a shape.

Calling event.preventDefault() in will-prevent-unload tells Electron to
ignore the iframe's beforeunload return value and proceed with the close.
The host app already persists diagrams via autosave + visibilitychange,
so the prompt was unnecessary.

Verified by reproducing the bug, applying the fix, and re-testing.

electron/main/window-manager.ts

@@ -60,6 +60,13 @@ export function createWindow(serverUrl: string): BrowserWindow {
         mainWindow.webContents.openDevTools()
     }
 
+    // Override the draw.io iframe's beforeunload handler so the window can
+    // close after the user edits text in a shape (fixes #815). Diagrams are
+    // already persisted via autosave, so the prompt is unnecessary.
+    mainWindow.webContents.on("will-prevent-unload", (event) => {
+        event.preventDefault()
+    })
+
     mainWindow.on("closed", () => {
         mainWindow = null
     })