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>

components/AccountSwitcher.tsx

@@ -1,5 +1,4 @@
 import { Ionicons } from "@expo/vector-icons";
-import { router } from "expo-router";
 import { useState } from "react";
 import {
   Modal,
@@ -12,7 +11,8 @@ import {
 
 import { fonts, radii, spacing } from "@/constants/theme";
 import { useAccounts } from "@/contexts/AccountsContext";
-import { useAuthClient, useSession } from "@/contexts/AuthContext";
+import { useSession } from "@/contexts/AuthContext";
+import { startAdditionalAccountSignIn } from "@/lib/add-account";
 import { useAppTheme } from "@/contexts/ThemeContext";
 import { formatServerHost } from "@/lib/server-mode";
 
@@ -34,7 +34,6 @@ function displayName(name: string, email: string) {
 /** Header control to switch signed-in accounts or add another. */
 export function AccountSwitcher() {
   const { colors } = useAppTheme();
-  const authClient = useAuthClient();
   const { data: session } = useSession();
   const {
     accounts,
@@ -56,9 +55,7 @@ export function AccountSwitcher() {
 
   async function handleAddAccount() {
     setOpen(false);
-    await authClient.signOut();
-    await clearActiveAccount();
-    router.replace("/(auth)/sign-in");
+    await startAdditionalAccountSignIn(clearActiveAccount);
   }
 
   async function handleSwitch(accountId: string) {