feat: add Electron app icon

Move the root logo into app assets, wire the Electron main window to use the branded icon, and stamp the packaged Windows executable icon with rcedit.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/main/appIcon.test.ts

@@ -0,0 +1,23 @@
+import { describe, expect, test } from 'bun:test';
+
+import { resolveWindowIconPath } from '@main/windows/appIcon';
+
+describe('resolveWindowIconPath', () => {
+  test('uses the Windows ico asset from the packaged app assets directory', () => {
+    expect(
+      resolveWindowIconPath({
+        appPath: 'C:\\workspace\\personal\\repositories\\kopaya',
+        platform: 'win32',
+      }),
+    ).toBe('C:\\workspace\\personal\\repositories\\kopaya\\assets\\icons\\app-icon.ico');
+  });
+
+  test('uses the PNG asset on non-Windows platforms', () => {
+    expect(
+      resolveWindowIconPath({
+        appPath: '/Applications/Kopaya.app/Contents/Resources/app',
+        platform: 'darwin',
+      }),
+    ).toBe('/Applications/Kopaya.app/Contents/Resources/app/assets/icons/app-icon.png');
+  });
+});