feat(accounts): implement user accounts, sessions, and dashboards

Introduce Stage 4 features to support multi-user accounts, cookie-based web sessions, and personal dashboards.

Changes include:
- Adding `/register` to bootstrap the first admin account and `/login`/`/logout` for session management.
- Creating a personal dashboard (`/app`) to display owned boxes, storage usage, and upload history.
- Implementing admin user management (`/admin/users`) for generating invite links and managing user states.
- Updating the bbolt database schema to store users, sessions, invites, and collections.
- Adding `golang.org/x/crypto` for password hashing and introducing unit tests for account handlers.

backend/go.mod

@@ -4,7 +4,8 @@ go 1.26
 
 require (
 	go.etcd.io/bbolt v1.4.3
+	golang.org/x/crypto v0.33.0
 	golang.org/x/image v0.41.0
 )
 
-require golang.org/x/sys v0.29.0 // indirect
+require golang.org/x/sys v0.30.0 // indirect