Stop using sessionStorage (security/usability issue)

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### Current Behavior:

UI uses window.sessionStorage for auth token

### Expected Behavior:

It should be stored in HTTP Only Cookie (better with secure flag).

### Why?

- Session Storage / Local storage isn't considered to be a secure storage for auth tokens: https://auth0.com/blog/secure-browser-storage-the-facts/
- Now you can't effectively work with Dependency Track using several tabs as each tab has it's own Session Storage. Many users think that it's a bug as absolute majority of the websites share auth session between tabs.

No files or tests are named. Start by locating the UI's window.sessionStorage usage and tracing the auth-token flow; verify how the frontend receives and sends authentication state. Done means authentication uses an HTTP-only, preferably secure cookie and the session is shared across browser tabs.

Written by the indexing model from the issue text.