Link upcoming events to their ICS URL

The "Nächste Veranstaltungen" table read the event URL via `ICAL.Event.url`,
but ical.js does not expose a `url` getter on `ICAL.Event` (it only has uid,
summary, description, color, location, sequence, the dates, organizer and
attendees). `event.url` was therefore always `undefined` and the `?? ""`
fallback turned it into an empty string, so every row rendered as
`<a href="">`, a dead link that just reloads the start page.

Read the URL from the VEVENT component instead. This also picks up the
`URL;VALUE=URI:` form used by most events in the published calendar, which is
exported from a CalDAV client and does not use a bare `URL:` property.

Not every event has a URL, so only wrap the name in a link when one is
present and emit plain text otherwise.

Checked against https://berlin.ccc.de/calendars/all.ics (31 events, 2 of them
without a URL):

  before: <td><a href="">CCCB Plenum</a></td>
  after:  <td><a href="https://wiki.berlin.ccc.de/Plenum">CCCB Plenum</a></td>
  after:  <td>Aktionstag gegen Überwachung im Chaos Computer Club Berlin</td>

Fixes: c28f04c6e8 ("switch to ics files; make calendars work; fix some minor issues")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

static/js/upcoming.js

@@ -7,7 +7,7 @@ import ICAL from "https://unpkg.com/ical.js/dist/ical.min.js";
  * @param {Date} now Events must start after this date
  * @param {number} maxEvents Maximum number of events to return
  * @param {number} maxDays Maximum number of days into the future
- * @returns {{start: Date, name: string, url: string}[]}
+ * @returns {{start: Date, name: string, url: string}[]} url is empty when the event has no URL
  */
 function getUpcomingEvents(icsText, now, maxEvents, maxDays) {
   const jcal = ICAL.parse(icsText);
@@ -25,6 +25,9 @@ function getUpcomingEvents(icsText, now, maxEvents, maxDays) {
       continue;
     }
 
+    // ICAL.Event does not expose the URL property, so read it from the component.
+    const url = component.getFirstPropertyValue("url") ?? "";
+
     if (event.isRecurring()) {
       const iterator = event.iterator();
 
@@ -47,7 +50,7 @@ function getUpcomingEvents(icsText, now, maxEvents, maxDays) {
           events.push({
             start,
             name: event.summary ?? "",
-            url: event.url ?? "",
+            url,
           });
         }
       }
@@ -58,7 +61,7 @@ function getUpcomingEvents(icsText, now, maxEvents, maxDays) {
         events.push({
           start,
           name: event.summary ?? "",
-          url: event.url ?? "",
+          url,
         });
       }
     }
@@ -100,11 +103,16 @@ document.addEventListener("DOMContentLoaded", () => {
 
         const colName = document.createElement("td");
 
-        const a = document.createElement("a");
-        a.href = event.url;
-        a.text = event.name;
+        if (event.url) {
+          const a = document.createElement("a");
+          a.href = event.url;
+          a.text = event.name;
+
+          colName.appendChild(a);
+        } else {
+          colName.innerText = event.name;
+        }
 
-        colName.appendChild(a);
         row.appendChild(colName);
 
         table.appendChild(row);