Add Departures screen (train departure board)

New screens/departures.py using the free, unofficial db-vendo-client REST
API (v6.db.transport.rest) -- no key needed, same "no API key" philosophy
as Weather's Open-Meteo integration. Community-run infra though, not
official DB, and it was actually down (503, then timing out entirely) for
the whole time I was building this -- couldn't live-verify against real
data the way Open-Meteo was earlier, so I built the parsing against the
project's documented example response and stress-tested render() with
mocked requests.get() covering on-time/delayed/cancelled/missing-platform/
very-long-direction cases instead. check() fails soft (logs a warning,
returns None) on any request error, matching the "service might be down"
reality of this API -- the screen shows a plain "not available" message
rather than erroring out.

Settings: station name (resolved to a station ID via /locations, same
pattern as Weather's city->coordinates geocoding), how many departures to
show (3/5/8), and its own configurable update interval via the
get_poll_interval() mechanism built for Weather (defaults to 2 min --
departure boards go stale much faster than weather does).

Caught two real layout bugs while stress-testing render() before shipping:
line names ("ICE 1234") and delayed/cancelled time text ("Fällt aus") were
both wider than the columns I'd budgeted for them, with the second one
overlapping the direction text when right-aligned past its column's own
left edge. Fixed by measuring actual worst-case widths at the chosen font
size instead of guessing, and truncating every field to its column as a
defensive backstop regardless.

web/app.py

@@ -98,6 +98,38 @@ def create_app(config, screens: dict, scheduler) -> Flask:
 
         return render_template("settings_weather.html", settings=config.get_screen_settings("weather"), error=error)
 
+    @app.route("/settings/departures", methods=["GET", "POST"])
+    def settings_departures():
+        error = None
+        if request.method == "POST":
+            station_query = request.form.get("station", "").strip()
+            results = int(request.form.get("results", 5) or 5)
+            if results not in (3, 5, 8):
+                results = 5
+            update_minutes = max(1, int(request.form.get("update_minutes", 2) or 2))
+
+            if station_query:
+                try:
+                    found = screens["departures"].find_station(station_query)
+                except requests.exceptions.RequestException:
+                    found = None
+                if found is None:
+                    error = f'Could not find a station matching "{station_query}" -- try a different spelling, or the service might be temporarily down.'
+                else:
+                    config.update_screen_settings("departures", {
+                        "station": station_query,
+                        "station_id": found["id"],
+                        "station_label": found["name"],
+                        "results": results,
+                        "update_minutes": update_minutes,
+                    })
+                    return redirect(url_for("settings_departures", saved=1))
+            else:
+                config.update_screen_settings("departures", {"results": results, "update_minutes": update_minutes})
+                return redirect(url_for("settings_departures", saved=1))
+
+        return render_template("settings_departures.html", settings=config.get_screen_settings("departures"), error=error)
+
     @app.route("/settings/picture", methods=["GET", "POST"])
     def settings_picture():
         error = None