* adds route at /api/auth/callback to be hit by the flask API at the completion of the login flow
* adds jwt token parsing logic
* adds support for a "SESSION_SECRET" env var
* adds a User page for temporary testing

* adds a context provider and hook to allow client components to access up to date logged in user information
* adds a NextJS route to return user data decrypted from the session cookie passed up from the client
* makes some temporary updates to the feature flags table in order for that page
to act as a proof of concept for the functionality
* moves some feature flags and loading spinner code to a more suitable location.

Creates a Node route for logging out to log out a user, which
* calls the API logout endpoint
* removes client side cookie

Also adds fetch function to use to call the API logout route and a dummy logout button to use for testing

…em for client side use (#3374)

* rewrites the FeatureFlagManager and useFeatureFlag hook to allow syncing flags between server and client using cookies
* adds the `authOn` feature flag in frontend code and terraform
* refactors the `environments` setup a bit to more easily expose feature flags
* splits functionality that does not benefit from being held in the FeatureFlagsManager class into a helper file
* moves feature flag manager file into a nested directory

* updates to session management to allow for decrypting the login.gov JWT in order to pull out user email
* creation of a logged in state for the header

## Summary
Fixes #2962

### Time to review: __15 mins__

## Changes proposed

* Adds a modal for the login link
* Moves the env route handler to a redirect


#### Mobile

![image](https://github.com/user-attachments/assets/4d44f545-f1ce-419e-898d-dde3d859f55b)

#### Desktop

![image](https://github.com/user-attachments/assets/a34ead51-57f7-4c9b-a2e4-631f58e085f3)


### TODO

- [x] verify it is accessible
- [x] add tests
- [x] look over again

* fix bug where mobile login dropdown appeared on top of mobile menu
* improve spacing on mobile login dropdown

* the callback route redirects to the home page on success, or an unauthorized page if no token is present or error page in error cases
* creates error and unauthorized pages
* adds middleware to implement the correct status codes on these redirects