Architecture for secure local services for an HTML5 app

I have users that want a multi-device app, with complete offline functionality.

Some of the data that needs to be stored offline is super duper hyper secret, and cannot be safely stored in HTML 5 local storage. Also some of the functionality is very complex (complex financial scenario modelling and visualisations)

My current idea for a solution to this is an HTML5 app with all relevant functionality against server side REST services, and then developing a local service layer for specific devices which stores and secures the super duper hyper secret data.

The app would then perform REST calls to the server when it is online, and when it is offline it would call those same interfaces on a local server.

The local server would upload data to the remote server (using those same calls) when it is online.

Questions: 1.) Any suggestions for javascript frameworks that could ease this setup? 2.) What back-end app architecture would work? I am torn between something enterprisey and easy with strong cryptography support (.NET or Java) vs something like node with mongodb.