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/src/routes/user_web.rs

@@ -270,7 +270,10 @@ struct DateGroup {
 #[derive(Template)]
 #[template(path = "my_cases.html")]
 struct MyCasesTemplate {
-    slug: String,
+    /// Human-readable label rendered in the `<h1>`. Resolved upstream
+    /// from `users.toml` (`display_name` field, falling back to `slug`)
+    /// so the template stays presentation-only.
+    display_name: String,
     groups: Vec<DateGroup>,
     total: usize,
     /// True iff the session user is an admin — toggles full case_id display.
@@ -572,16 +575,17 @@ pub async fn handle_my_cases(
     // flip (new transcripts in, stale document, ...).
     auto_trigger::try_enqueue_all_for_user(&user_root, &pipeline.analyze_tx, &events_tx).await;
 
+    // Single users.toml lookup feeds both the retention sweep below and
+    // the per-user preview-lines cap further down. Cheaper than walking
+    // `config.users` twice for the same slug on every page render.
+    let user_record = config.users.iter().find(|u| u.slug == user.slug);
+    let retention = user_record.map(|u| u.retention.clone()).unwrap_or_default();
+    let preview_lines = user_record.map(|u| u.preview_lines).unwrap_or(2);
+
     // Lazy retention sweep: at most one disk scan per /web/cases visit,
     // reusing the user's retention policy from users.toml. Runs before
     // the listing scan so any auto-close/auto-purge actions are
     // reflected in the same response.
-    let retention = config
-        .users
-        .iter()
-        .find(|u| u.slug == user.slug)
-        .map(|u| u.retention.clone())
-        .unwrap_or_default();
     crate::retention::sweep_user_retention(&user_root, &user.slug, &retention, &events_tx).await;
 
     let show_closed = query.include_closed();
@@ -610,15 +614,9 @@ pub async fn handle_my_cases(
     };
     let groups = group_by_utc_date(cases, &closed_extra);
     let is_admin = user.is_admin();
-    let preview_lines = config
-        .users
-        .iter()
-        .find(|u| u.slug == user.slug)
-        .map(|u| u.preview_lines)
-        .unwrap_or(2);
     MyCasesTemplate {
         csrf_token: user.csrf_token,
-        slug: user.slug,
+        display_name: user.display_name,
         groups,
         total,
         is_admin,