refactor(desktop): extract instance-URL helpers to instanceUrl.ts

Pure mechanical extraction — no behavior change. Enables recovery.ts
to share these helpers without a circular import on main.ts.

packages/desktop/src/instanceUrl.ts

@@ -0,0 +1,32 @@
+import { app } from 'electron';
+import path from 'path';
+import fs from 'fs';
+
+export function getInstanceUrlPath(): string {
+  return path.join(app.getPath('userData'), 'instance-url.json');
+}
+
+export function loadInstanceUrl(): string | null {
+  try {
+    const data = JSON.parse(fs.readFileSync(getInstanceUrlPath(), 'utf-8'));
+    return typeof data.url === 'string' ? data.url : null;
+  } catch {
+    return null;
+  }
+}
+
+export function saveInstanceUrl(url: string): void {
+  fs.writeFileSync(getInstanceUrlPath(), JSON.stringify({ url }));
+}
+
+export function clearInstanceUrl(): void {
+  try {
+    fs.unlinkSync(getInstanceUrlPath());
+  } catch {
+    // File may not exist — ignore
+  }
+}
+
+export function getPickerPath(): string {
+  return path.join(__dirname, '..', 'resources', 'instance-picker.html');
+}