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/auth-storage.ts

@@ -41,6 +41,33 @@ export async function migrateAuthStorage(fromPrefix: string, toPrefix: string):
   );
 }
 
+export async function clearAuthStorage(prefix: string): Promise<void> {
+  await Promise.all(
+    AUTH_STORAGE_SUFFIXES.map(async (suffix) => {
+      const key = storageKeyForPrefix(prefix, suffix);
+      const value = await SecureStore.getItemAsync(key);
+
+      if (value?.startsWith(CHUNK_MARKER)) {
+        const count = Number(value.slice(CHUNK_MARKER.length));
+        if (Number.isInteger(count) && count > 0) {
+          await Promise.all(
+            Array.from({ length: count }, (_, index) =>
+              SecureStore.deleteItemAsync(`${key}.${index}`),
+            ),
+          );
+        }
+      }
+
+      await SecureStore.deleteItemAsync(key);
+    }),
+  );
+}
+
+/** Clears guest auth storage before signing into an additional account. */
+export async function prepareForAdditionalSignIn(): Promise<void> {
+  await clearAuthStorage(GUEST_AUTH_STORAGE_PREFIX);
+}
+
 export async function finalizeAuthenticatedAccount(input: {
   apiUrl: string;
   userId: string;