Fix onboarding UX and improve extension performance

[stackedPenguin]

Summary

This PR addresses multiple UX and performance issues in the wallet onboarding and extension popup experience.

Changes

Onboarding Improvements

• ✅ Add hidden "Advanced" section in setup screen with import seed phrase option
• ✅ Remove unnecessary "Import Wallets" screen when creating new wallet
• ✅ Fix wallet import flow to show recovery phrase input instead of skipping to account selection

Performance Improvements

• ✅ Disable Backpack API feature gates fetch (404 error) - use local defaults instead
• ✅ Improve localStorage caching for user data to persist across popup opens
• ✅ Fix slow popup opening issue (now opens in <1 second)

UI/UX Fixes

• ✅ Fix black screen issue by showing loading skeleton instead of null
• ✅ Suppress React Native BackHandler warning in web environment

Files Changed

• packages/app-extension/src/components/Onboarding/pages/CreateOrImportWallet.tsx
• packages/app-extension/src/components/Onboarding/pages/OnboardAccount.tsx
• packages/app-extension/src/gates/FEATURES.ts
• packages/recoil/src/atoms/secure-client/userClientAtoms.ts
• packages/app-extension/src/app/Router.tsx
• packages/app-extension/src/index.tsx
• packages/app-extension/src/refactor/screens/Unlocked/Settings/WalletAddBackpackRecoveryPhraseScreen.tsx

Test Plan

• Tested wallet creation flow - no unnecessary import screens
• Tested wallet import from "Advanced" section during setup
• Tested wallet import from settings - recovery phrase input now shows
• Tested popup opening performance - opens quickly
• Verified no black screens during loading
• Verified no console errors or warnings

Impact

All changes improve the user experience by making onboarding smoother, eliminating unnecessary steps, and significantly improving popup performance.