feat: Display most recent recording time

Add `recorded_at_iso` and `time_hms_utc` to `CasePageTemplate` to
display the timestamp of the most recent recording.
Introduce a new partial `partials/time_format.js` for consistent date
and time formatting in the browser.
Update templates to use the new partial for rendering timestamps and
group headings.

server/src/routes/user_web.rs

@@ -134,6 +134,12 @@ struct CasePageTemplate {
     case_id: String,
     case_id_short: String,
     oneliner: Option<String>,
+    /// RFC3339 UTC timestamp of the most recent recording. `None` iff the
+    /// case has no recordings at all. Drives the datetime header under the
+    /// title; browser JS formats it to "Heute 11:34" / "13.12.2022 11:34".
+    recorded_at_iso: Option<String>,
+    /// UTC `HH:MM` as a no-JS fallback inside the `<time>` element.
+    time_hms_utc: String,
     /// Rendered document HTML (already passed through `md_to_html`). `Some`
     /// iff `document.md` exists at render time — template prioritises this
     /// over status/analyzing placeholders.
@@ -315,10 +321,16 @@ pub async fn handle_case_page(
     let case_id_short = case_id_str.chars().take(8).collect();
     let is_admin = user.is_admin();
 
+    let most_recent = recordings.last().map(|r| r.filename.as_str());
+    let recorded_at_iso = most_recent.map(recorded_at_iso_of).filter(|s| !s.is_empty());
+    let time_hms_utc = most_recent.map(extract_hhmm_utc).unwrap_or_default();
+
     CasePageTemplate {
         case_id: case_id_str,
         case_id_short,
         oneliner,
+        recorded_at_iso,
+        time_hms_utc,
         document_html,
         recordings_count,
         transcribed_count,