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/users.toml.example

@@ -8,6 +8,11 @@ slug = "dr_mueller"
 api_key = "change-me-to-a-secure-key"
 web_password = "$2b$12$..."
 role = "doctor"
+  # Optional: human-readable name shown in the web UI page header
+  # in place of the slug (the slug stays the on-disk directory and
+  # login identifier). Free-form UTF-8 — accents and spaces are fine.
+  # Defaults to the slug when omitted.
+  display_name = "Dr. Müller"
   # Optional: how many visual lines of the LLM analysis preview the case
   # list renders under each row. The preview itself is always the first
   # paragraph of document.md; this value only caps the visible lines