feat: finish native customization and platform seams

Summary

• add supported post-generate hooks and tab-selection appearance configuration for generated iOS/Android projects
• let native iOS screens opt into app light/dark appearance and keep it synchronized at runtime
• document the customization contract and cover it in generator and appearance tests
• implement the macOS WKWebView open-panel delegate so <input type=file> works
• reject stale watch state payloads and keep the iOS config validator in sync with nested tab-bar configuration

Closes #389 Closes #388 Closes #400 Related to #344

Verification

• zig build test
• bun run test:sdk
• bun run typecheck
• focused iOS and Android generator suites
• focused Pickier and git diff --check

The full mobile simulator/emulator matrix is left to GitHub Actions.