feat: expose profile import over IPC

Wire the existing profiles::import_profile through the JSON-lines IPC
protocol so the GUI can add an existing account: ImportProfile request
and handler in src/ipc.rs, import_profile added to the Electron method
allowlist, api/AppProvider importProfile, and the Add existing account
buttons in ProfilesScreen.

Also add the active signing identity card on Home and fix the HomeScreen
tests whose text queries now match the identity card as well as the
profile row.

frontend/electron/main.ts

@@ -195,6 +195,7 @@ const RENDERER_METHODS: ReadonlySet<string> = new Set([
   'init',
   'get_state',
   'create_profile',
+  'import_profile',
   'select_profile',
   'publish_profile_metadata',
   'set_profile_picture',