Vendor ical.js instead of loading it from unpkg.com

The upcoming events table pulled its ICS parser straight from a CDN with
`import ICAL from "https://unpkg.com/ical.js/dist/ical.min.js"`. That sends
every visitor of the start page to unpkg.com, which hands their IP address and
user agent to a third party before any of our own code runs. The URL is not
even pinned to a version, so whatever ical.js publishes next is executed on our
site without anybody looking at it, and the start page silently breaks when the
CDN is unreachable.

Check the parser into `assets/js/vendor/` and let Hugo bundle it. This is what
`js.Build` is for: it runs the esbuild that is built into Hugo, so it resolves
the import at build time and needs no node_modules and no extra tooling in the
build environment. The result is minified and fingerprinted like the other
scripts of the site, and the script tag carries a subresource integrity hash.

Since the bundle now has a content hash in its name, its URL cannot be written
by hand in the markdown any more. Move the table and the script tag into an
`upcoming` shortcode, which is the same pattern `calendar.html` already uses,
and move `upcoming.js` from `static/` to `assets/` so Hugo can process it.

The vendored file is the unminified `dist/ical.js` of the pinned release: it
carries the MPL-2.0 header and is the source form of what we ship, and Hugo
minifies it for delivery anyway. `assets/js/vendor/README.md` records the
version, where it came from and how to update it.

The built bundle renders the same table as before, checked against
https://berlin.ccc.de/calendars/all.ics.

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>

README.md

@@ -30,7 +30,7 @@ This is the website of the CCCB.
 Every change you make on the project will be reflected in your browser as long as `hugo serve` is running.
 
 The *"Nächste Veranstaltungen"* table on the home page and the calendar under `/verein/calendar/` are rendered in the
-browser by `static/js/upcoming.js` and `assets/js/calendar.js`. Both fetch `/calendars/all.ics`, which is **not**
+browser by `assets/js/upcoming.js` and `assets/js/calendar.js`. Both fetch `/calendars/all.ics`, which is **not**
 generated by Hugo and is not part of this repo — it is published separately on the web server. Without it both tables
 stay empty. Download the published calendar into `static/`, which Hugo serves under the same path, and they work
 locally, under `hugo serve` as well as in a built site:
@@ -46,6 +46,9 @@ are currently published.
 Hugo does generate an `.ics` feed per section (for example `/veranstaltungen/index.ics`) from the `dtstart`, `dtend`
 and `rrule` front matter of the pages in `content/veranstaltungen/`, using the `.ics` templates under `layouts/`.
 
+The site must not make the browser load anything from an external server, so third party JavaScript is checked into
+`assets/js/vendor/` and bundled in by Hugo instead of being pulled from a CDN. See the README there before updating it.
+
 To build the site for upload, run:
 
 ```shell

assets/js/upcoming.js

@@ -1,4 +1,4 @@
-import ICAL from "https://unpkg.com/ical.js/dist/ical.min.js";
+import ICAL from "./vendor/ical.js";
 
 /**
  * Read the URL of an event.

assets/js/vendor/README.md

@@ -0,0 +1,19 @@
+# Vendored JavaScript
+
+Third party code is checked in here instead of being loaded from a CDN, so that the website does not make the visitor's
+browser fetch anything from an external server.
+
+Hugo bundles and minifies these files into the scripts that reference them, so the unminified source is checked in.
+
+## ical.js
+
+- Version: 2.2.1
+- Source: <https://unpkg.com/ical.js@2.2.1/dist/ical.js>
+- Upstream: <https://github.com/kewisch/ical.js>
+- License: MPL-2.0 (see the header of `ical.js`)
+
+To update, download the `dist/ical.js` of the wanted release and replace the file, keeping the version above in sync:
+
+```shell
+curl -o assets/js/vendor/ical.js https://unpkg.com/ical.js@<version>/dist/ical.js
+```

content/_index.md

@@ -25,9 +25,6 @@ description: "Startseite CCCB mit Kurzkalender"
 
 ### Nächste Veranstaltungen
 
-<table id="upcoming" class="table table-condensed">
-</table>
+{{< upcoming >}}
 
 Weitere Termine findest du im [Veranstaltungskalender](/verein/calendar/).
-
-<script type="module" src="/js/upcoming.js"></script>

layouts/shortcodes/upcoming.html

@@ -0,0 +1,3 @@
+{{- $js := resources.Get "js/upcoming.js" | js.Build (dict "minify" true "format" "esm" "target" "es2020") | fingerprint -}}
+<table id="upcoming" class="table table-condensed"></table>
+<script type="module" src="{{ $js.RelPermalink }}" integrity="{{ $js.Data.Integrity }}"></script>