feat: Add display name to user profiles

Introduce `display_name` field to `User` struct for human-readable names
in the UI.
This falls back to the user's `slug` if not provided.

Update `AuthenticatedWebUser` and `MyCasesTemplate` to use this new
field for rendering.
Add tests for parsing `display_name` and ensure fallback logic works
correctly.
Update `users.toml.example` to document the new field.

server/tests/common/users.rs

@@ -33,6 +33,7 @@ pub fn test_user(slug: &str) -> User {
         api_key: format!("key-{slug}"),
         web_password: hash_password(TEST_PASSWORD),
         role: "doctor".into(),
+        display_name: None,
         language: None,
         retention: Default::default(),
         window_hours: 72,