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/TopChromeBar.tsx

@@ -45,11 +45,7 @@ export function TopChromeBar() {
 
 const styles = StyleSheet.create({
   host: {
+    flexShrink: 0,
     overflow: "hidden",
-    position: "absolute",
-    top: 0,
-    left: 0,
-    right: 0,
-    zIndex: 10,
   },
 });