Add Authentik sign-in, fix tab scroll insets, and polish multi-account auth.

Mobile app detects SSO per server, supports OAuth sign-in, and preserves saved
sessions when adding accounts. Tab screens get proper chrome layout and tab-bar
clearance with scrollable page headers.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/add-account.ts

@@ -0,0 +1,12 @@
+import { router } from "expo-router";
+
+import { prepareForAdditionalSignIn } from "@/lib/auth-storage";
+
+/** Switch to guest mode and open sign-in without wiping other saved accounts. */
+export async function startAdditionalAccountSignIn(
+  clearActiveAccount: () => Promise<void>,
+) {
+  await clearActiveAccount();
+  await prepareForAdditionalSignIn();
+  router.replace("/(auth)/sign-in");
+}