feat: support updating Garmin credentials at runtime

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/garmin/mock/mock.go

@@ -18,6 +18,8 @@ type Client struct {
 	authResultCursor   int
 	ClosedCalled       bool
 	GetActivitiesCalls int
+	LastEmail          string
+	LastPassword       string
 }
 
 var _ garmin.Client = (*Client)(nil)
@@ -70,6 +72,11 @@ func (c *Client) GetActivityDetails(ctx context.Context, activityID int64) (garm
 	return c.Details[activityID], nil
 }
 
+func (c *Client) UpdateCredentials(email, password string) {
+	c.LastEmail = email
+	c.LastPassword = password
+}
+
 func (c *Client) Close() error {
 	c.ClosedCalled = true
 	return nil