feat: add unified error handling with AuthError class

[Ray0907]

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc
  annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and
  libraries)
• ⚠️ Breaking change (fix or feature that would cause existing
  functionality to change)

📚 Description

Adds a unified error handling system for both local and authjs
providers.

Problem

Previously, error handling was inconsistent across the module:

• Some errors were silently ignored
• Some threw exceptions
• Some returned undefined
• Error formats varied between providers

Solution

• Add AuthError class with structured error data (code, message,
  statusCode, recoverable, timestamp)
• Add error, setError, clearError to useAuth() and useAuthState()
  return values
• Add createAuthError factory functions for common error types
• Add toAuthError() converter for normalizing any error to AuthError
• Export error utilities via auto-imports and #auth module

Usage

const { signIn, error, clearError } = useAuth()

await signIn(credentials)

if (error.value) {
  console.log(error.value.code)    // 'INVALID_CREDENTIALS'
  console.log(error.value.message) // 'Invalid username or password'

  if (error.value.recoverable) {
    // show retry button
  }
}

Available Error Codes

INVALID_CREDENTIALS | INVALID_PROVIDER | TOKEN_EXPIRED | TOKEN_INVALID |
TOKEN_MISSING | TOKEN_PARSE_ERROR | SESSION_EXPIRED | SESSION_FETCH_ERROR |
NETWORK_ERROR | TIMEOUT | SERVER_ERROR | ENDPOINT_DISABLED | UNKNOWN_ERROR

📝 Checklist

- I have linked an issue or discussion.
- I have added tests (if possible).
- I have updated the documentation accordingly.

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