hello Ananth! That sounds like an interesting project.
As far as I understand, in my capacity as a non-lawyer who cannot offer official legal advice, your path forward looks like this:
1. Make sure you are compliant with the licenses of your dependencies
If you have already published your app, but are not sure about this, get the license text for each dependency to your employer’s legal counsel and ask for their opinion. This is something you should do regardless of the decision to open-source.
2. Decide why you want to publish your code
Most employers will not see open-sourcing projects for which they paid as intrinsically good. You will need convince them why open-sourcing this project at all is good for them before you pick a license. The specific license matters much, much less than this act of convincing.
One common argument, which I generally agree with, is that publishing open-source code will raise the company’s profile for the purposes of recruitment. People will be able to see what kind of work you do, and that you’re willing to open source it. It may follow that their code written for you will be open-sourced, which can be nice to put on a future resume. If you think that your code is high-quality enough as it is, and would only need to add license text and turn it public on Github, this is a nice way in.
I also see a common argument that I do not agree with: that any open-source code will benefit from community bug fixes and features. This is only true if people have both the skills and motivation to improve your code, or if you create that motivation (say, by hiring them or by cultivating an enthusiastic community around your product—a full-time job.) You may see this if you open-source a library, but it probably will not happen for a website.
3. Decide on a license for the code you wrote
Github has a guide on choosing a license at https://choosealicense.com/. Ask your counsel about their preferences for licensing; you can go through that site together and have a discussion after you convince them that open-source is a good idea here.
The Elm community commonly uses the BSD 3-clause license (the three clauses being that redistributions in source and binary forms must retain the license, the author(s) names cannot be used as an endorsement of derivatives, and that the software comes without a warranty and with a waiver of liability.)
Best of luck!