fix: connect frontend auth and profile flows to backend endpoints

[kckagancan]

Summary

This PR connects the frontend auth and profile flows to real backend endpoints and removes remaining local demo-only behavior in these areas.

What changed

• connected login, signup, verify email, resend verification, and current-user flows to backend APIs
• added token-based session handling and working logout behavior
• connected complete-profile and profile view/save flows to backend fetch/patch endpoints
• updated verify-email flow to use tokenized email links instead of a 6-digit code flow
• improved validation, empty states, and error handling across auth/profile forms
• added remember-me compatible token persistence behavior in the auth layer

Notes

• profile updates still happen through multiple PATCH requests, so partial-save risk is communicated in the UI where relevant
• document upload UI remains placeholder-only until backend upload support is available

Can you please review? @erinc00

Closes #132

[erinc00]

Looks good to me overall. I reviewed the latest changes and everything seems in good shape from my side. The implementation is consistent with the current structure, the flow is clear, and I do not see any major issue blocking merge. Approving. Ready to merge.