Add user-facing error messages to Swift API discovery

## Summary

The Kotlin wrapper has `ApiDiscoveryResult.userFacingErrorMessage(url:)` that maps API discovery failures to human-readable strings (offline, invalid SSL, not a WordPress site, etc.). The Swift side currently surfaces raw `error.localizedDescription` in the example app's `LoginView`, which produces unhelpful messages.

## Context

Added in PR #1190 for Kotlin:
- `native/kotlin/api/kotlin/src/main/kotlin/rs/wordpress/api/kotlin/ApiDiscoveryResult.kt`

We should align this approaches so they're similar. Kotlin's intermediate layer is much smaller, but we probably want to derive the user-facing error in Rust and share the approach.

## Files to change

- `native/swift/Sources/wordpress-api/` — add error extension with user-facing messages
- `native/swift/Example/Example/UI/LoginView.swift` — use the new messages in `handleLoginError()`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start with native/kotlin/api/kotlin/src/main/kotlin/rs/wordpress/api/kotlin/ApiDiscoveryResult.kt to understand the existing user-facing error mappings. Then inspect native/swift/Sources/wordpress-api/ and Example/Example/UI/LoginView.swift, especially handleLoginError(). Done means Swift discovery failures produce human-readable messages instead of raw error.localizedDescription, with the approach aligned across platforms.

Written by the indexing model from the issue text.