feat(P3.T3): app:hostname IPC for device-name default

apps/desktop/electron/modules/app-hostname.ts

@@ -0,0 +1,21 @@
+// app:hostname adapter — returns the OS hostname via Node's `os.hostname()`.
+// The renderer has no Node access (contextIsolation is ON), so it must ask
+// main for this value. Used by AuthContext on first launch to populate the
+// `devices` row with a human-readable default device name.
+
+import os from 'node:os';
+
+import { ipcMain } from 'electron';
+
+import { CHANNELS } from '../ipc-types';
+
+export function register(): void {
+  ipcMain.handle(CHANNELS.APP_HOSTNAME, (): string | null => {
+    try {
+      const h = os.hostname();
+      return typeof h === 'string' && h.length > 0 ? h : null;
+    } catch {
+      return null;
+    }
+  });
+}