Fix garth token serialization using TypeAdapter for Pydantic dataclasses

Garth's OAuth1Token and OAuth2Token are Pydantic dataclasses, not BaseModel
subclasses, so they require TypeAdapter for serialization instead of model_dump().
Also adds user-friendly error handling for authentication failures.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

scripts/garmin_auth.py

@@ -10,11 +10,15 @@ Usage:
     python3 garmin_auth.py
 """
 import json
+import sys
 from datetime import datetime
 from getpass import getpass
 
 try:
     import garth
+    from garth.auth_tokens import OAuth1Token, OAuth2Token
+    from garth.exc import GarthHTTPError
+    from pydantic import TypeAdapter
 except ImportError:
     print("Error: garth library not installed.")
     print("Please install it with: pip install garth")
@@ -24,11 +28,25 @@ email = input("Garmin email: ")
 password = getpass("Garmin password: ")
 
 # MFA handled automatically - prompts if needed
+try:
     garth.login(email, password)
+except GarthHTTPError as e:
+    if "401" in str(e):
+        print("\nError: Invalid email or password.", file=sys.stderr)
+    else:
+        print(f"\nError: Authentication failed - {e}", file=sys.stderr)
+    exit(1)
+except Exception as e:
+    print(f"\nError: {e}", file=sys.stderr)
+    exit(1)
+
+# Serialize Pydantic dataclasses using TypeAdapter
+oauth1_adapter = TypeAdapter(OAuth1Token)
+oauth2_adapter = TypeAdapter(OAuth2Token)
 
 tokens = {
-    "oauth1": garth.client.oauth1_token.model_dump(),
-    "oauth2": garth.client.oauth2_token.model_dump(),
+    "oauth1": oauth1_adapter.dump_python(garth.client.oauth1_token, mode='json'),
+    "oauth2": oauth2_adapter.dump_python(garth.client.oauth2_token, mode='json'),
     "expires_at": garth.client.oauth2_token.expires_at
 }