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/tab-layout.ts

@@ -6,12 +6,13 @@ import { spacing } from "@/constants/theme";
 export const tabLayout = StyleSheet.create({
   pageHeader: {
     gap: 4,
+    paddingTop: spacing.md,
+    paddingBottom: spacing.md,
   },
   scrollContent: {
     paddingHorizontal: spacing.md,
   },
   scrollBody: {
     gap: spacing.md,
-    marginTop: spacing.sm,
   },
 });