Any leg to stand on?

Ownership of Intelectual Property (aka ideas) should have been detailed in your employment contract, particularly if access to your IP is what they hired you for. If not, you're in the clear.

Alternatively, they may have an IP clause in their corporate code of conduct.

You are entitled to a copy of anything and everything you put your signature on. You should obtain copies and have a solicitor specialising in IP have a look over them if the sections pertaining to IP are unclear or confusing.

It is unethical and immoral, but yes they can.

Unless you have copyrights on materials or patents on inventions, when they employ you they are allowed to take advantage of any ideas you willingly bring to the table while being paid to perform your skills. Technically, people are usually hired for the skills and ideas they bring to the table that someone else couldn't.

Because you are uninqe and have great ideas, people want you. Be flattered and go find something better there's no use looking back, instead just use that as a positive point at the next interview. You can say how your ideas helped improve operations, save money or whatever the benefit was. Document such things and say how much things improved/sales increased etc. due to your ideas.

I'd be flattered not ticked off. :)

This is not the first time I've heard of this.
I'm not into web design, but a buddy of mine was working on a CBL (computer based learning) program for a local company here in town. He acquired the contract and spent about four weeks designing and creating the web based program.
The prog was meant to train and test new workers in the area of industrial safety and the laws that apply.
One day, the customer approached the boss and asked to see the work so far...boss gave him the code...and my buddy subsequently lost the contract.
One month of hard work and thousands of dollars stolen out of my friends pocket.

Never trust your employer...business is business.
Always keep one eye open.

Did you write down and publish your ideas prior to your starting with the company? Copyright tends to protect you from this kind of nonsense.